Refactor RequestDelegateGenerator to use partial class pattern

[Copilot]

Refactor RequestDelegateGenerator to use partial class pattern

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Restructure RequestDelegateGenerator to match ValidationsGenerator and OpenApiGenerator patterns

Description

Refactors RequestDelegateGenerator to use the same partial class structure as ValidationsGenerator (src/Validation/gen) and XmlCommentGenerator (src/OpenApi/gen).

Changes

• RequestDelegateGenerator.cs — Now contains only the Initialize method (pipeline setup)
• RequestDelegateGenerator.Parser.cs (new) — Parsing logic:
  • IsEndpointInvocation — Predicate for identifying Map* invocations
  • TransformEndpoint — Creates endpoint models from syntax contexts
• RequestDelegateGenerator.Emitter.cs (new) — Emission logic:
  • EmitInterceptorDefinition — Generates interceptor method code
  • EmitHttpVerbs, EmitEndpointHelpers, EmitHelperTypes — Collect and emit supporting code
  • Emit — Final source output

Structure

Before:
  RequestDelegateGenerator.cs (300+ lines, all inline)

After:
  RequestDelegateGenerator.cs        (~60 lines, pipeline only)
  RequestDelegateGenerator.Parser.cs (~30 lines)
  RequestDelegateGenerator.Emitter.cs (~260 lines)

No functional changes — existing tests pass unchanged.

Original prompt

Rewrite the RequestDelegateGenerator implementation to mimic the structure of the ValidationsGenerator in src/Validation/gen and the OpenAPI generator in src/OpenApi/gen.

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