feat: add MongoDB driver handshake metadata

[alexbevi]

Adds a rig_mongodb::client_options() helper that returns ClientOptions pre-configured with MongoDB DriverInfo, allowing the MongoDB server to identify traffic from Rig applications in server-side telemetry, Atlas dashboards, and db.currentOp().

What changes

rig-mongodb receives a pre-built mongodb::Collection from callers, so it has no hook point to inject metadata at construction time. The Rust driver has no post-construction API equivalent, so the right approach is a public factory helper:

// Before
let options = ClientOptions::parse(uri).await?;
let client = Client::with_options(options)?;

// After
let options = rig_mongodb::client_options(uri).await?;
let client = Client::with_options(options)?;

The helper sets driver_info with name "rig-mongodb" and the crate's version (read at compile time via env!("CARGO_PKG_VERSION")). The existing example in examples/vector_search_mongodb.rs is updated to use the helper.

How it appears in the logs

The handshake document visible in mongod diagnostic logs and db.currentOp() will include a driver field alongside the base mongodb Rust driver info:

{
  "driver": {
    "name": "mongodb|rig-mongodb",
    "version": "3.7.0|0.40.0"
  }
}

See the MongoDB handshake specification for details.

Testing

Added two unit tests in src/lib.rs:

• client_options_sets_driver_info — confirms driver_info.name equals "rig-mongodb" and version is present
• driver_name_is_not_empty — guards against accidental empty-string name

cargo test -p rig-mongodb  # 4 tests pass

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• I have added tests that prove my feature works
• New and existing unit tests pass locally with my changes