Implement native MongoDB TLS transport for secret-gated integration

Follow-up to #284.

The corpus can now load and correctly skip js/third_party/mongodb/mongodb.test.ts when TLS_MONGODB_DATABASE_URL is absent, with a structured credential-safe fallback for unsupported connection attempts. Completing the live integration still requires a real MongoDB driver transport in Home rather than a bootstrap simulation.

Acceptance criteria

• connect to a TLS MongoDB endpoint supplied through TLS_MONGODB_DATABASE_URL
• implement client connect/close and database collection selection
• support insertOne, cursor/find inspection, and deleteOne used by the Bun test
• redact credentials from all messages and stacks
• retain operation, sanitized endpoint, error code, and original network/TLS cause on failures
• run the secret-gated Bun integration successfully when credentials are available