Replace the per-thread cached read connections with a pool. Each
execute_fn()/execute() callback checks a connection out for its duration
and gets a LeasedConnection proxy that raises ConnectionLeaseError if
used after the callback returns.
New settings: max_open_connections (global LRU cap, default 128),
connection_idle_timeout (seconds, default 60, enforced by one
process-wide reaper thread plus opportunistic reaping on checkout),
connection_pool_wait_ms (soft cap: wait then exceed) and
pool_read_connections (off = fresh connection per query, a control).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwkLo2rJWidpkDVT3HkvNv
