feat(ts): ts_error_handling=result — typed Result return for es-mode client

[SebastienMelki]

Summary

This is @hishamank's work from #210, rebased cleanly onto main after #207 was squash-merged (which deleted the base branch and auto-closed #210).

All code authored by @hishamank — original PR: #210. Previously approved by @SebastienMelki.

Changes

• Adds ts_error_handling=result plugin option (requires ts_runtime=protobuf-es)
• Unary methods return Promise<Result<T, ClientError>> instead of throwing
• ClientError = discriminated union of ValidationError | ApiError | <every proto *Error type>
• Structural error registry matches response JSON keys against proto Error schemas
• Zero-field *Error guard: keys.length > 0 prevents empty errors from matching everything
• SSE methods still throw (can't return Result from AsyncGenerator)
• 3 lint findings cleared (funlen, goconst, perfsprint)

Test plan

• CI passes (first-ever CI run for this code)
• Golden files match expected output
• go test ./internal/tsclientgen/... ./internal/tscommon/... ./internal/urlparamtest/... all pass

🤖 Generated with Claude Code

[github-actions]

🔍 CI Pipeline Status

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

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🔗 Artifacts: Test results and coverage reports uploaded

[codecov]

Codecov Report

❌ Patch coverage is 18.18182% with 135 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.64%. Comparing base (c432ae9) to head (b48eaf2).

Files with missing lines	Patch %	Lines
internal/tscommon/result.go	0.00%	109 Missing ⚠️
internal/tsclientgen/generator.go	36.00%	14 Missing and 2 partials ⚠️
internal/tscommon/imports.go	0.00%	8 Missing ⚠️
internal/tsclientgen/modules.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
- Coverage   28.80%   28.64%   -0.16%     
==========================================
  Files          73       74       +1     
  Lines       12890    13048     +158     
==========================================
+ Hits         3713     3738      +25     
- Misses       8989     9119     +130     
- Partials      188      191       +3     

Flag	Coverage Δ
unittests	28.64% <18.18%> (-0.16%)	⬇️

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