Add connect_timeout option to ConnectionOptions

[nlively]

Adds a connect_timeout: Option<Duration> field to ConnectionOptions that bounds how long TCP/TLS connection establishment can take. Without this, connection attempts to unreachable hosts block indefinitely (or until OS-level TCP timeouts kick in, typically 30–120s), which makes it difficult for callers to implement responsive failure handling.

Changes

• Added connect_timeout field to ConnectionOptions with documentation
• Applied the timeout in Connection::connect_once() for direct (non-DNS) connections
• Threaded through dns::build_endpoint() and dns::spawn_dns_reresolution() so DNS-balanced connections also respect the timeout on initial connect and during background re-resolution
• Added a test that verifies a 500ms timeout properly bounds a connection attempt to a non-routable IP

Usage

let opts = ConnectionOptions::new(url)
    .connect_timeout(Duration::from_millis(5_000))
    .build();
let conn = Connection::connect(opts).await?;

When connect_timeout is None (the default), behavior is unchanged — no explicit timeout is applied.

[CLAassistant]

All committers have signed the CLA.