…onnectionRetryable

Eliminates per-instance __dict__ allocation and defers the _deprioritized_servers
list creation until it is actually needed (only on sharded retry paths).

Constructing _PoolCheckout directly bypassed pool.checkout() overrides
used in MockPool, causing mock_down_hosts checks to be skipped and
network-error tests to fail.

…rver.checkout()

server.checkout() was a thin wrapper around pool.checkout() that nothing
called after _ClientCheckout was changed to call server.pool.checkout()
directly.

Add a test that mocks publish_connection_checked_out to raise and verifies
the connection is returned to the pool rather than leaked.

…connection

Covers the except BaseException block in _ClientCheckout.__aenter__() that
fires when post-checkout work (session pinning, ConfigurationError check)
raises — verifying the connection is returned to the pool and not leaked.

…thon

patch.object cannot shadow a method on an instance when __slots__ is
defined — the attribute is read-only. Use a subclass override instead.

Resolve import conflict in server.py: upstream removed datetime import
(run_operation timing moved into run_cursor_command), our branch removed
AbstractAsyncContextManager (server.checkout() was deleted). Drop both
now-unused imports.

- Remove _checkout_started_time from _PoolCheckout.__slots__: the value
  was written then immediately aliased to a local variable and never read
  from the slot again, wasting an allocation on every checkout.

- Guard pool_checkout.__aexit__ with try/finally in both _ClientCheckout
  __aexit__ and the __aenter__ setup-failure path: if handle() raises,
  the connection is now guaranteed to be checked in instead of leaked.

- Pass exc.__traceback__ instead of None when calling
  pool_checkout.__aexit__ from the __aenter__ setup-failure except block.

- Reset conn.pinned_txn/pinned_cursor before calling pool_checkout.__aexit__
  in the setup-failure except block so _PoolCheckout.__aexit__ calls checkin()
  instead of silently skipping it when session._pin() ran before the error.
- Replace _ClientReadCheckout._effective_read_pref slot with _read_preference
  and a local variable in __aenter__ to avoid mutating instance state.
- Fix Pool.checkout() docstring to say "with-statement" so synchro.py copies
  the correct text to the sync mirror.