fix: thread conn= into failover timeout paths; invalidate pooled connections in tracker

[AhmadMasry]

Description

Follow-up to #1252 (sync foundation for asyncio support, phase 1 of #1251). This is sync-side hardening of the failover paths, intended to land before the async phase (phase 2) is introduced.

Two changes:

1. Thread conn= into the failover plugins' timeout paths. feat: sync foundation for asyncio support (phase 1 of #1251) #1252 introduced the abort-and-drain mechanism for timed-out offloaded operations (on timeout: shut down the operation's socket via driver_dialect.abort_connection and wait for its worker thread, so a later close/reuse of the connection cannot race the still-running operation) and applied it to the host-monitoring and limitless paths. This PR passes conn= to driver_dialect.execute() in the failover plugins' rollback/close call sites (failover_plugin.py, failover_v2_plugin.py), completing the mechanism for the failover paths.

2. Prefer PoolProxiedConnection.invalidate() over close() in the connection tracker. When wrapper-internal connection pools (SqlAlchemyPooledConnectionProvider) are enabled, the connections tracked by OpenedConnectionTracker are SQLAlchemy pool proxies. Calling close() on a proxy checks it back into the pool, which first runs rollback-on-return against the failed writer — an unbounded blocking call on the tracker thread when the host is unreachable — and re-pools the connection if that rollback happens to succeed (e.g. the old writer came back as a reader). invalidate() (public SQLAlchemy API) skips the rollback and discards the connection, so the pool opens a fresh connection on the next checkout. Plain driver connections keep the existing close() behavior.

Unit tests cover both the invalidate-vs-close dispatch and the end-to-end pool behavior against a real QueuePool (connection discarded without rollback-on-return, fresh connection on next checkout).

Proposed CHANGELOG entry (under ### :bug: Fixed):

Failover plugins now pass the target connection into the execute-timeout path so timed-out rollback/close operations abort their socket and drain their worker; the Aurora connection tracker now invalidates (rather than re-pools) connections from wrapper-internal connection pools after writer failover. (PR #1255)

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