feat: runtime response-schema validation in dev/test

[ethanj]

Summary

Closes the gap Codex flagged: `check:openapi` only catches schema↔spec drift, not formatter↔schema drift. A formatter change that forgets to update the schema would still slip past the gate. This PR adds a dev/test-mode middleware that wraps `res.json()` and parses the outgoing body against the route's declared response schema — prod is a pass-through no-op, so zero per-request cost in production.

What's in the PR

• `src/middleware/validate-response.ts` — the wrapper. Throws loudly in dev/test when a 2xx body doesn't match its schema; no-op when `NODE_ENV=production`. Error messages point at the right fix (update the formatter, or update the schema + regenerate the spec).
• `src/routes/response-schema-map.ts` — the route→schema map. Keyed by Express's router-relative `${method} ${route.path}` so parameterized paths like `/:id` resolve correctly.
• Wired into both `createMemoryRouter` and `createAgentRouter` via `router.use(validateResponse(...))` before any handler.

Real drift caught on first run (both fixed here)

1. `Date` leaking into search memory `created_at`. Express serializes `Date` → ISO string, so the wire shape is string, but `res.json()` sees the raw Date before serialization. Added an `IsoDateString` preprocess that accepts both.
2. `NaN` leaking into search similarity/score/importance. `JSON.stringify(NaN)` writes `null` on the wire, so the wire shape is `number | null`, but `res.json()` sees NaN. Added a `NumberOrNaN` preprocess that converts NaN → null and validates as nullable number.

Schemas now report accurate wire types (string for dates, nullable number for NaN-possible floats). OpenAPI spec regenerates with zero further diff.

Test plan

• `npx tsc --noEmit` clean
• All 1037 core tests pass with the middleware active
• `npm run generate:openapi` produces zero diff after the schema fixes
• Verified the validator throws on drift — the Date and NaN issues above were caught by it on the first test run
• CI green end-to-end