Fixes #1093

Summary

• Reclaim a non-idle connection when no remaining pool request owns it.
• Apply the same cleanup invariant to the synchronous and asynchronous pools.
• Add an async regression that cancels a request after assignment but before dispatching it to the connection.

Root cause

A cancelled async request can be removed after receiving a newly created connection but before calling that connection. The connection is neither idle nor closed, and no longer belongs to any request, so it remains in the pool and consumes a connection slot indefinitely.

Validation

• python -m ruff format httpcore tests --check
• python -m ruff check httpcore tests
• python -m mypy httpcore/_async/connection_pool.py httpcore/_sync/connection_pool.py tests/_async/test_connection_pool.py
• python -m pytest -q (216 passed, 6 xpassed)