refactor: add @_disfavoredOverload to QueryExpression methods conflicting with stdlib

- Add @_disfavoredOverload to QueryExpression.map() (Optional.swift:273)
- Add @_disfavoredOverload to QueryExpression.flatMap() (Optional.swift:293)
- Remove deprecated .count property that conflicted with Collection.count

These changes resolve ambiguity when QueryExpression extensions are imported
alongside Swift standard library types, ensuring the compiler prefers stdlib
methods for regular Swift types while keeping SQL query building methods
available for QueryExpression types.

This helps packages importing StructuredQueriesPostgres avoid type inference
conflicts when working with regular Swift collections and optionals.

All 835 tests passing.

Author: coenttb

Sources/StructuredQueriesCore/Optional.swift

@@ -270,6 +270,7 @@ extension QueryExpression where QueryValue: _OptionalProtocol {
     ///
     /// - Parameter transform: A closure that takes an unwrapped version of this expression.
     /// - Returns: The result of the transform function, optionalized.
+    @_disfavoredOverload
     public func map<T>(
         _ transform: (SQLQueryExpression<QueryValue.Wrapped>) -> some QueryExpression<T>
     ) -> some QueryExpression<T?> {
@@ -289,6 +290,7 @@ extension QueryExpression where QueryValue: _OptionalProtocol {
     ///
     /// - Parameter transform: A closure that takes an unwrapped version of this expression.
     /// - Returns: The result of the transform function.
+    @_disfavoredOverload
     public func flatMap<T>(
         _ transform: (SQLQueryExpression<QueryValue.Wrapped>) -> some QueryExpression<T?>
     ) -> some QueryExpression<T?> {

Sources/StructuredQueriesPostgres/Functions/String/BasicStringFunctions.swift

@@ -30,15 +30,6 @@ extension QueryExpression where QueryValue: Collection {
     public func length() -> some QueryExpression<Int> {
         QueryFunction("length", self)
     }
-
-    @available(
-        *,
-        deprecated,
-        message: "Use 'count()' for SQL's 'count' aggregate function, or 'length()'"
-    )
-    public var count: some QueryExpression<Int> {
-        length()
-    }
 }
 
 extension QueryExpression where QueryValue == String {