feat: Add XML docs and Obsolete annotations

[razor-x]

Motivation

• Emit C# XML documentation comments and [Obsolete] attributes in generated SDK code by using schema prose and deprecation reasons so consumers get richer API docs and deprecation guidance.

Description

• Add a documentation Handlebars partial and include it in templates to render /// <summary> XML docs and [Obsolete] attributes before classes, enums, properties, and API methods using new helpers.
• Introduce csDoc and csString helpers in handlebars-helpers.ts to escape and format schema text for C# XML comments and string literals, and wire them into templates.
• Extend the model builder in build-model.ts to carry documentation and obsoleteMessage fields (and richer enum value metadata via EnumValue) through normalization and class/enum/route construction so templates can render docs and deprecations; update class-model.ts types accordingly.
• Regenerate C# output templates to include documentation everywhere (many output/csharp/src/Seam/Api/*.cs files updated accordingly).

Testing

• Regenerated the C# SDK sources with the updated generator and verified the files were produced with XML docs and [Obsolete] annotations (diff contains many updated output/csharp files).
• Ran the TypeScript build and tests (npm run build and npm test) for the codegen package and confirmed they succeeded with no type errors.

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Codex Task