fix(sse): preserve Copilot discovered endpoint routing

[zebrajaeger]

Summary

• Fixes incorrect endpoint selection for newly imported GitHub Copilot models that only support the Responses API. These models could previously fall back to /chat/completions and fail with 400 ... not accessible via the <path>.
• Preserves advertised endpoints as supportedEndpoints and sets targetFormat: "openai-responses" for Responses-only models.
• Normalizes endpoint paths for the existing synced-model schema and public model catalog.
• Preserves existing routing for chat-capable models and the Claude/Gemini Responses-endpoint safeguards. No blanket GPT-version heuristic is introduced.

Related Issues

• No linked issue.

Validation

Validation follows the
Contribution Golden Path:

• Change type: provider / routing
• Focused tests and category gates from the golden path
  • All four focused GitHub test suites pass: 42 tests total.
  • Additional provider category gates have not been run locally.
• npm run lint
  • Targeted ESLint and Prettier checks for all changed files passed, along with the commit hooks.
• Reconciled with the current active release base; focused checks rerun afterward
  • Based on release/v3.8.52. Focused tests passed again after local integration.
  • A fresh reconciliation with the current remote release base is still pending.
• Production-code changes include a new or updated automated test in this PR
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Additional commands run:

• npm run typecheck:core — passed.
• npm run test:coverage — coverage thresholds met; exited with code 1 due to failures elsewhere in the full test run, including missing better-sqlite3 and failing Windows certificate stubs.
• npm run test:vitest — MCP audit failures; these were also reproduced on the unchanged base branch.

Production build validation and full CI approval are pending.

Tests Added Or Updated

• tests/unit/github-copilot-model-discovery.test.ts — updated:
  • Preservation and normalization of Responses-only endpoint metadata.
  • Nested endpoint declarations and deduplication.
  • Conservative behavior when metadata is incomplete.
• tests/unit/github-copilot-discovered-routing.test.ts — added:
  • Discovery → synced-model persistence → runtime format resolution → GitHub executor URL.
  • Responses-only, chat-only, dual-endpoint, and metadata-free models.
  • Claude/Gemini safeguards and VS Code model-catalog compatibility.

The existing suites in github-copilot-custom-model-target-format.test.ts and executor-github.test.ts were also run successfully.

Coverage Notes

The production change is limited to
open-sse/services/githubCopilotModels.ts.

Report	Statements	Lines	Functions	Branches
Changed module	79.40%	79.40%	75.00%	80.00%
Overall report	80.80%	80.80%	82.97%	79.76%

All measured categories exceed the 60% minimum.
The full coverage gate is nevertheless not green because of the test failures noted above.

A before/after coverage comparison was not performed; no coverage improvement over the base is claimed.

Reviewer Notes

• No database migrations, new dependencies, feature flags, or credential changes.
• Previously imported GitHub models must be re-imported to populate the endpoint metadata that was formerly discarded.
• Endpoint selection remains provider-scoped and metadata-driven.
• Local OpenCode configuration changes are not included in this PR.