Simplify single-backend HttpApi exerciser

[kitlangton]

Summary

• Remove the single-value Backend parameter from the HttpApi exercise harness.
• Rename trace/auth diagnostics from effect ... to backend-neutral request/response wording.
• Keep coverage/auth/effect modes and route scenarios unchanged.

Testing

• bun typecheck from packages/opencode
• bunx prettier --check packages/opencode/test/server/httpapi-exercise/backend.ts packages/opencode/test/server/httpapi-exercise/runner.ts packages/opencode/test/server/httpapi-exercise/types.ts
• bunx oxlint packages/opencode/test/server/httpapi-exercise/backend.ts packages/opencode/test/server/httpapi-exercise/runner.ts packages/opencode/test/server/httpapi-exercise/types.ts
• bun run script/httpapi-exercise.ts --mode effect --fail-on-missing --fail-on-skip printed summary pass=145 fail=0 skip=0 missing=0 extra=0; the process then remained alive until the shell timeout, matching the existing script resource-lifetime behavior.

Stack

1. Clean up post-Hono references #26903
2. Simplify single-backend HttpApi exerciser #26906 👈 current
3. Drop unused small ID Zod statics #26908