Rename message_base to header for terminology clarity

[Copilot]

• Explore repository structure and understand the scope of changes
• Clean up build artifacts
• Rename message_base.hpp to header.hpp
• Rename message_base.cpp to header.cpp
• Update class definition from message_base to header
• Update all includes of message_base.hpp to header.hpp
• Update all references to message_base class in code
• Update test file message_base.cpp to header.cpp
• Update documentation references
• Update the main header boost/http_proto.hpp
• Rename variable m to h for header parameters
• Fix documentation inline code references
• Verify no compilation errors

Summary

This PR completes the terminology refactor requested in the issue by renaming message_base to header throughout the codebase. This clarifies that the class represents the start-line plus fields (the HTTP header), not the full message including body.

Changes Made

Files Renamed

• include/boost/http_proto/message_base.hpp → include/boost/http_proto/header.hpp
• src/message_base.cpp → src/header.cpp
• test/unit/message_base.cpp → test/unit/header.cpp

Classes Renamed

• class message_base → class header

Variables Renamed

• All function parameters header const& m → header const& h
• All function body usages of m → h where referring to header
• All documentation inline code references from `m` → `h`

Updated Files

• include/boost/http_proto.hpp - Updated include directive
• include/boost/http_proto/fields_base.hpp - Updated friend declaration
• include/boost/http_proto/request_base.hpp - Updated base class and constructor calls
• include/boost/http_proto/response_base.hpp - Updated base class and constructor calls
• include/boost/http_proto/serializer.hpp - Updated forward declaration, function signatures, and documentation
• include/boost/http_proto/impl/serializer.hpp - Updated template function parameters and usages
• src/serializer.cpp - Updated all references to use new class name and variable name
• test/unit/metadata.cpp - Updated test lambdas with new type
• test/unit/request.cpp - Updated include directive
• doc/modules/ROOT/pages/reference.adoc - Updated documentation reference

Impact

This is a breaking change for any code that directly references the message_base class. Users will need to update their code to use header instead. The change is purely a rename with no functional changes to the API.

Verification

• No message_base references remain in source code (verified via grep)
• All include guards updated appropriately
• All inheritance relationships preserved
• All function signatures updated consistently
• All variable names updated from m to h for header parameters
• All documentation updated including inline code references

Original prompt

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This section details on the original issue you should resolve

<issue_title>terminology refactor: header</issue_title>
<issue_description>We use the term "message" to describe the start-line plus fields, and this is confusing because we also use "message" to describe the full request or response including the body. I propose the following, which we should discuss first:

1. Repurpose the term "header" to mean start-line plus fields
2. Always say "fields" instead of "headers" (never say "headers")
3. Use "request header" to mean a request-line plus fields
4. Use "response header" to mean a status-line plus fields
5. Rename message_base to header
   </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #175

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[vinniefalco]

@copilot the variable should be renamed from m to h, and this should happen everywhere that a message_base variable is changed to be a header

[Copilot]

Done in commit 6b19f77. Renamed all header const& m parameters to header const& h and updated all usages throughout the serializer code.