Retire stateful connections without reset proof

[mjc]

Summary

• classify client and backend exits explicitly in the stateful loop
• finalize every pooled lease on success and error paths
• preserve clean completed responses while retiring dirty or incomplete connections

Branch-added tests

• Concurrent stateful clients exercise pooled connection retirement and verify the pool does not retain closed sockets.
• Pool status coverage checks that explicit retirement removes the object without shrinking unrelated capacity.
• Response-transfer cases distinguish clean completed client disconnects from queued bytes, backend EOF, and I/O failure.

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[Copilot]

🟡 Changes recommended

The new stateful error-finalization path has a non-exhaustive match (won’t compile) and the hybrid/stateful error wrapping can misclassify client disconnect/write errors as backend errors, impacting runtime logging/behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR refines stateful connection lifecycle handling so pooled backend leases are always finalized, and backend reuse/retirement decisions are based on explicit session-exit classification and “clean vs dirty” response-transfer outcomes.

Changes:

• Adds explicit stateful session exit/disposition tracking and uses it to decide whether to reuse or retire pooled backend connections.
• Adjusts response-transfer semantics so a completed response with no queued backend bytes can keep the backend reusable even if the final client write fails.
• Prevents retired deadpool objects from re-entering the idle pool by taking the object before dropping, and extends pool-status coverage in tests.

File summaries

File	Description
src/session/response_transfer.rs	Updates backend reuse decision for ClientWrite when the connection is otherwise reusable, and updates tests accordingly.
src/session/handlers/stateful.rs	Introduces explicit exit/disposition types and finalizes pooled connection guards on all paths (but currently has a blocking error-path issue).
src/session/handlers/hybrid.rs	Centralizes backend lease finalization via a new finalize() helper and applies stateful-loop disposition to pooled lease retirement.
src/pool/provider.rs	Ensures retired connections cannot be returned to deadpool by dropping Object::take(conn) and extends test assertions around pool size.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite

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