feat: add C# and Python generators and extend TS client generation

[elzalem]

Summary

This PR adds two new Sebuf generators and extends the TypeScript client generator for a few missing contract shapes.

Changes included here:

• add protoc-gen-csharp-http
• add protoc-gen-py-client
• add a shared internal contract model used by the new generators
• extend protoc-gen-ts-client to support message-only proto files
• add field_names=json|proto support for TS client output
• fix TS client generation for empty request messages

Why

Sebuf already supports Go and TypeScript generation, but it was missing generator coverage for C# and Python clients.

While integrating Sebuf into a downstream project, a few TS generator gaps also surfaced:

• some proto packages only define messages and still need generated client-side model output
• some consumers need proto field names rather than JSON/camel names
• empty request RPCs could produce awkward or invalid generated code paths

These changes make Sebuf usable across more client stacks without relying on repo-local custom generators.

What Changed

New generators

• cmd/protoc-gen-csharp-http
• cmd/protoc-gen-py-client

These generators emit contract/model code from the same proto inputs Sebuf already handles for other languages.

Shared contract traversal

• internal/contractmodel

This consolidates message/enum/service traversal logic so the new generators do not duplicate structure walking.

TypeScript generator improvements

• message-only proto support
• field_names=json|proto option
• empty-request handling fix

These changes are covered by updated golden tests.

Tests

Added or updated tests for:

• C# generator golden output
• Python generator golden output
• TS generator golden output for message-only protos
• TS generator output for proto field naming
• existing TS generator paths affected by empty request handling

[github-actions]

🔍 CI Pipeline Status

✅ Lint: success
✅ Test: success
✅ Coverage: success
✅ Build: success
✅ Integration: success

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📊 Coverage Report: Available in checks above
🔗 Artifacts: Test results and coverage reports uploaded

[codecov]

Codecov Report

❌ Patch coverage is 0% with 265 lines in your changes missing coverage. Please review.
✅ Project coverage is 2.99%. Comparing base (917521f) to head (dca9c89).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/pyclientgen/generator.go	0.00%	106 Missing ⚠️
internal/csharpgen/generator.go	0.00%	103 Missing ⚠️
internal/tscommon/types.go	0.00%	28 Missing ⚠️
internal/tsclientgen/generator.go	0.00%	19 Missing ⚠️
internal/tsclientgen/types.go	0.00%	8 Missing ⚠️
internal/tsservergen/generator.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #126      +/-   ##
========================================
- Coverage   3.17%   2.99%   -0.19%     
========================================
  Files         47      49       +2     
  Lines       7485    7941     +456     
========================================
  Hits         238     238              
- Misses      7243    7699     +456     
  Partials       4       4              

Flag	Coverage Δ
unittests	2.99% <0.00%> (-0.19%)	⬇️

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