Cancel connection attempts after queued requests abort

[dills122]

Summary

Fixes #386 by rejecting requests that abort while connection establishment is still pending and cancelling connection attempts once no queued request needs them.

Previously, abort handling was attached when a request reached onRequestStart. A request waiting behind an unresolved connector had not reached that point, so aborting its signal did not settle the request until the connector eventually succeeded or failed.

What Changed

• Reject pre-aborted requests without starting a connection.
• Bind the existing public API abort listener to requests while they are still queued.
• Cover request, stream, pipeline, connect, and upgrade consistently.
• Give each request-driven connection attempt its own AbortSignal and abort it after every request depending on that attempt has been removed.
• Preserve intentional internal preconnect attempts that have no queued request.
• Ignore late callbacks from cancelled attempts and destroy any stale socket they return.
• Keep intentional cancellation silent instead of emitting connectionError and causing pools to evict an otherwise usable client.
• Document and type the optional connector signal.

Validation

• npm run lint
• npm run test:typescript
• Full test/*.js unit corpus
• Full test/node-test/**/*.js corpus
• Full test/interceptors/*.js corpus
• Focused request, stream, pipeline, connect, upgrade, pool, preconnect, and pipelining regression suites
• 11 new tests covering exact abort reasons, listener cleanup, shared attempts, retry composition, stale connector callbacks, built-in connector cancellation, and pool lifecycle behavior

Scope Notes

Custom connectors receive the attempt-scoped signal and should stop pending work when it aborts. A connector that ignores the signal cannot have its internal work forcibly stopped, but Undici still settles the request promptly and prevents a late socket or error from affecting a newer attempt.

The cancellation signal represents the lifetime of a connection attempt, not any single request. A shared attempt is cancelled only when all queued requests relying on it are gone.