[codex] Cover formatter safety edge cases

[vku2018]

Summary

• Extend format.test.ts with the remaining safeStringify edge cases requested in Add unit tests for safeStringify circular, BigInt, and truncation handling #146: BigInt inside arrays, custom maxChars, and exact-limit payloads.
• Add a dashed invalid timestamp-string fallback assertion for safeFormatTimestamp.
• Keep this as a test-only change; no production code needed.

Refs #146

Validation

• npm test -- src/lib/tests/format.test.ts (25 passing)
• npx eslint src/lib/tests/format.test.ts
• git diff --check (passes with existing CRLF warning only)
• npx jest src/lib/tests/format.test.ts --coverage --collectCoverageFrom=src/lib/format.ts --coverageThreshold "{}" (format.ts: 100% statements, 93.18% branches, 100% functions, 100% lines)
• npm run typecheck (blocked by pre-existing missing imports in src/app/agents/[agent]/page.tsx and src/app/docs/page.tsx)