fix: concurrent conn checkouts can continue after pool close

[quettabit]

closes #119

[greptile-apps]

Greptile Summary

This PR prevents connection establishment from outliving connection-pool shutdown.

• Tracks tasks that are creating and checking out new connections.
• Re-checks pool closure before creating or registering a connection.
• Cancels and awaits active connection-establishment tasks during close.
• Closes partially initialized connections that never become pool-owned.

Confidence Score: 5/5

The PR appears safe to merge; concurrent connection establishment is cancelled and cleaned up before pool shutdown completes.

No actionable failure remains: checkout tasks are registered without an intervening suspension, closure is re-checked before pool ownership is committed, and both partial and pool-owned connections have deterministic cleanup paths.

Important Files Changed

Filename	Overview
src/s2_sdk/_client.py	Adds lifecycle tracking, cancellation, and partial-connection cleanup so concurrent checkouts cannot continue beyond pool shutdown.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant Pool as ConnectionPool
    participant Task as Checkout task
    participant Conn as Connection

    Caller->>Pool: checkout(base_url)
    Pool->>Pool: acquire host lock and check _closed
    Pool->>Task: create and track task
    Task->>Conn: connect()
    Caller->>Pool: close()
    Pool->>Pool: "set _closed = true"
    Pool->>Task: cancel()
    Task->>Conn: close if not pool-owned
    Pool->>Pool: await checkout tasks
    Pool->>Conn: close pool-owned connections
    Pool-->>Caller: close completes

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