Add default value support for all primitive types in elicitation schemas (SEP-1034) (#924)

* Initial plan

* Add SEP-1034 support for default values in all primitive schema types

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Address PR feedback: convert if-else to switch and add default value mapping tests

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Remove default value extraction code and linker annotations per feedback

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Use literal true/false values instead of GetBoolean() call

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Reinstate tests and fix CreatePrimitiveSchema to extract defaults from AttributeProvider

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Remove manual default extraction and update test to use positional record

Removed the manual default value extraction code from BuildRequestSchema
and SetDefaultValue helper as this should be handled by AIJsonUtilities.
Updated FormWithDefaults to be a positional record with optional parameters.
Test is now skipped until AIJsonUtilities supports extracting defaults from
optional parameters.

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Use 'is not null' pattern for consistency with nullable checks

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -109,6 +109,8 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                 double? minimum = null;
                 double? maximum = null;
                 bool? defaultBool = null;
+                double? defaultNumber = null;
+                string? defaultString = null;
                 IList<string>? enumValues = null;
                 IList<string>? enumNames = null;
 
@@ -158,7 +160,23 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                             break;
 
                         case "default":
-                            defaultBool = reader.GetBoolean();
+                            // We need to handle different types for default values
+                            // Store the value based on the JSON token type
+                            switch (reader.TokenType)
+                            {
+                                case JsonTokenType.True:
+                                    defaultBool = true;
+                                    break;
+                                case JsonTokenType.False:
+                                    defaultBool = false;
+                                    break;
+                                case JsonTokenType.Number:
+                                    defaultNumber = reader.GetDouble();
+                                    break;
+                                case JsonTokenType.String:
+                                    defaultString = reader.GetString();
+                                    break;
+                            }
                             break;
 
                         case "enum":
@@ -188,7 +206,8 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                             psd = new EnumSchema
                             {
                                 Enum = enumValues,
-                                EnumNames = enumNames
+                                EnumNames = enumNames,
+                                Default = defaultString,
                             };
                         }
                         else
@@ -198,6 +217,7 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                                 MinLength = minLength,
                                 MaxLength = maxLength,
                                 Format = format,
+                                Default = defaultString,
                             };
                         }
                         break;
@@ -208,6 +228,7 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                         {
                             Minimum = minimum,
                             Maximum = maximum,
+                            Default = defaultNumber,
                         };
                         break;
 
@@ -265,6 +286,10 @@ public override void Write(Utf8JsonWriter writer, PrimitiveSchemaDefinition valu
                         {
                             writer.WriteString("format", stringSchema.Format);
                         }
+                        if (stringSchema.Default is not null)
+                        {
+                            writer.WriteString("default", stringSchema.Default);
+                        }
                         break;
 
                     case NumberSchema numberSchema:
@@ -276,10 +301,14 @@ public override void Write(Utf8JsonWriter writer, PrimitiveSchemaDefinition valu
                         {
                             writer.WriteNumber("maximum", numberSchema.Maximum.Value);
                         }
+                        if (numberSchema.Default is not null)
+                        {
+                            writer.WriteNumber("default", numberSchema.Default.Value);
+                        }
                         break;
 
                     case BooleanSchema booleanSchema:
-                        if (booleanSchema.Default.HasValue)
+                        if (booleanSchema.Default is not null)
                         {
                             writer.WriteBoolean("default", booleanSchema.Default.Value);
                         }
@@ -296,6 +325,10 @@ public override void Write(Utf8JsonWriter writer, PrimitiveSchemaDefinition valu
                             writer.WritePropertyName("enumNames");
                             JsonSerializer.Serialize(writer, enumSchema.EnumNames, McpJsonUtilities.JsonContext.Default.IListString);
                         }
+                        if (enumSchema.Default is not null)
+                        {
+                            writer.WriteString("default", enumSchema.Default);
+                        }
                         break;
 
                     default:
@@ -371,6 +404,10 @@ public string? Format
                 field = value;
             }
         }
+
+        /// <summary>Gets or sets the default value for the string.</summary>
+        [JsonPropertyName("default")]
+        public string? Default { get; set; }
     }
 
     /// <summary>Represents a schema for a number or integer type.</summary>
@@ -399,6 +436,10 @@ public override string Type
         /// <summary>Gets or sets the maximum allowed value.</summary>
         [JsonPropertyName("maximum")]
         public double? Maximum { get; set; }
+
+        /// <summary>Gets or sets the default value for the number.</summary>
+        [JsonPropertyName("default")]
+        public double? Default { get; set; }
     }
 
     /// <summary>Represents a schema for a Boolean type.</summary>
@@ -456,5 +497,9 @@ public IList<string> Enum
         /// <summary>Gets or sets optional display names corresponding to the enum values.</summary>
         [JsonPropertyName("enumNames")]
         public IList<string>? EnumNames { get; set; }
+
+        /// <summary>Gets or sets the default value for the enum.</summary>
+        [JsonPropertyName("default")]
+        public string? Default { get; set; }
     }
 }