fix: harden AG-UI recorder, mock, and handler error handling

[jpr5]

Summary

Pre-existing hardening items surfaced during CR on PR #149.

• agui-recorder.ts: Log client write failures (was bare catch), add console.warn fallback for parseSSEEvents when no logger, forward upstream content-type on non-2xx instead of SSE headers
• agui-mock.ts: Wrap readBody in try/catch (was unguarded await), default logger to "warn" instead of "silent"
• agui-types.ts: Add logLevel option to AGUIMockOptions
• logger.ts: Add "warn" log level with proper hierarchy (silent < warn < info < debug)
• cli.ts + types.ts: Accept "warn" in LogLevel union and CLI validation

Test plan

• 2682 tests pass
• Prettier clean
• ESLint clean
• tsc --noEmit clean