Source diff message query pattern

[kitlangton]

Summary

• Remove the GET /session/{sessionID}/diff messageID query schema override from public.ts.
• Cover that the query parameter pattern now comes from the runtime MessageID schema.

Testing

• bun test --timeout 5000 test/server/httpapi-query-schema-drift.test.ts
• bun typecheck from packages/opencode
• bunx prettier --check packages/opencode/src/server/routes/instance/httpapi/public.ts packages/opencode/test/server/httpapi-query-schema-drift.test.ts
• bunx oxlint packages/opencode/src/server/routes/instance/httpapi/public.ts packages/opencode/test/server/httpapi-query-schema-drift.test.ts
• ./packages/sdk/js/script/build.ts generated only unrelated declaration-order churn; not committed.

Stack

• Source HTTP API ID path patterns #26623
• Source workspace path pattern #26632
• Remove redundant ID Zod overrides #26633
• Source diff message query pattern #26638 ← current