Implement POST /auth/google/disconnect handler (LEY-16)

[ltyu]

Summary

Implements the POST /auth/google/disconnect endpoint which previously returned 501. Authenticated users can now revoke their Gmail connection.

Changes

• Replace 501 stub in src/handlers/disconnectGoogleOAuth.ts with a full implementation:
  • Uses JwtAuthenticatedAppUserProvider to extract userId from the verified JWT sub claim
  • Calls gmailConnectionRepository.clearRefreshToken() to null the encrypted token and set status to revoked — preserves the record for auditing
  • Returns 200 on success, 404 when no connection exists (ConditionalCheckFailedException), 401 when unauthenticated
  • Rethrows unexpected errors for Lambda error handling
• Follows the same dependency-injection pattern as startGoogleOAuth and googleOAuthCallback
• Add src/handlers/disconnectGoogleOAuth.test.ts with 4 tests: 401 unauthenticated, 200 success with correct DynamoDB call, 404 not found, rethrow on unexpected error

Test results

Test Files  14 passed (14)
     Tests  45 passed (45)

pnpm exec tsc --noEmit passes clean.

Closes LEY-16