fix: conns that received GOAWAY during checkout are added to the pool

[quettabit]

closes #106

[greptile-apps]

Greptile Summary

This PR prevents a connection that received HTTP/2 GOAWAY during setup from entering the connection pool.

• Closes the affected connection and reports a connection-level error before reserving a request stream.
• Clarifies setup failure messages and consistently classifies zero initial stream capacity as a connection error.
• Updates the client test fixture and zero-capacity assertion for the revised behavior.

Confidence Score: 5/5

The PR appears safe to merge, with no actionable correctness, security, or repository-rule violations identified.

The new guard closes and rejects a GOAWAY-marked connection before it is pooled or assigned a stream, while the accompanying exception changes preserve the relevant internal retry semantics.

Important Files Changed

Filename	Overview
src/s2_sdk/_client.py	Adds a pre-pooling GOAWAY guard and refines connection-establishment errors without introducing a confirmed behavioral regression.
tests/test_client.py	Extends mocked connections with GOAWAY state and updates the zero-capacity exception expectation.

_{Reviews (1): Last reviewed commit: "initial commit" | Re-trigger Greptile}