Add a thread-local cache disable pool test (depends on database-connector#26)

[marknefedov]

Blocked on duckdb/database-connector#26 — CI will be red until that merges and the submodule pointer is bumped.

What this is

The SQL-level regression test for the thread-local connection cache leak. The fix itself is in the database-connector submodule (duckdb/database-connector#26); only the test belongs in this repo.

The bug

TryAcquireFromThreadLocal stops consulting the cache as soon as tl_cache_enabled goes false, but a connection already parked in that thread's cache is left there. It still counts towards total_connections while being unreachable, so it permanently occupies a pool slot:

Connection pool timeout: all 1 connections in use, waited 100ms

Reachable from this repo through postgres_configure_pool(enable_thread_local_cache := false) on a pool that had the cache enabled and in use.

The test

test/sql/storage/attach_connection_pool_tl_cache_disable.test walks enable → park → shrink → disable → query.

Two things worth flagging for review:

• It sets threads=1. The connection has to be parked and re-acquired on the same thread for the drain to fire, since the fix only recovers the calling thread's slot.
• It runs a control query first with the cache off and max_connections=1. If a single connection were not enough for this query, the later assertion could not distinguish a stranded connection from a query that simply needs two — the control makes that failure mode explicit rather than silent.

The pool also has a test/test_pool.cpp unit test in the connector PR, which is the more direct regression. Note that database-connector/test is not wired into this repo's CI, so that one only runs in the connector repo.

To land

1. Merge Return the parked connection when the thread-local cache is disabled database-connector#26
2. Bump the database-connector submodule pointer on this branch
3. CI goes green, undraft