fix: conns opened while closing the pool are not closed

[quettabit]

closes #97

[greptile-apps]

Greptile Summary

Fixes a connection-pool shutdown race by checking whether the pool closed before publishing a newly opened connection.

• Closes the new connection and raises S2ClientError when shutdown wins the race.
• Avoids temporarily adding zero-capacity connections to the pool.
• Updates the zero-stream-capacity test to reflect the revised bookkeeping.

Confidence Score: 5/5

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

The shutdown-race fix closes a connection before raising when the pool has closed and prevents rejected zero-capacity connections from entering pool state. The prior test-coverage thread was resolved after quettabit dismissed it, explaining that additional tests are left to human discretion.

Important Files Changed

Filename	Overview
src/s2_sdk/_client.py	Adds an atomic pre-publication lifecycle check that closes a newly opened connection when pool shutdown wins the race.
tests/test_client.py	Updates the zero-capacity assertion because rejected connections are no longer inserted into the host map.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant Pool
    participant Connection
    participant Shutdown

    Caller->>Pool: checkout(base_url)
    Pool->>Connection: open and negotiate SETTINGS
    Shutdown->>Pool: close()
    Pool->>Pool: mark closed
    Connection-->>Pool: negotiation completes
    Pool->>Pool: _add_connection()
    alt Pool is closed
        Pool->>Connection: close()
        Pool-->>Caller: raise S2ClientError
    else Pool remains open
        Pool->>Pool: publish connection
        Pool-->>Caller: return checkout
    end

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_{Reviews (2): Last reviewed commit: "initial commit" | Re-trigger Greptile}