test(opencode): lock BadRequest HttpApi error contracts

[TimothyVang]

Summary

• Extend public OpenAPI tests so experimental, tui, instance, event, and provider routes keep documented 400 BadRequest contracts from PRs fix(opencode): declare BadRequest errors on provider and project-copy routes #6–fix(opencode): declare BadRequest on remaining HttpApi group routes #8.
• Keep the structural source scan that requires every HttpApiEndpoint in groups/ to declare error:.
• Scanned all HttpApi groups: no endpoints missing error: (no production churn).

Test plan

• bun typecheck in packages/opencode
• monorepo pre-push bun turbo typecheck (29 packages)
• bun test test/server/httpapi-public-openapi.test.ts test/server/httpapi-error-contracts-source.test.ts
• bun test test/server/ (305 pass, 2 skip)

Notes

• Prefer tests over endpoint churn; no TUI rebrand.
• Public OpenAPI still intentionally strips 400 on POST /experimental/console/switch via normalizeLegacyOperation (legacy SDK surface).

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[TimothyVang]

Superseded by #11 (same OpenAPI locks + error-contract runner).