Add deterministic connection pool fuzzer

[arturobernalg]

This change adds a new deterministic fuzzer-style test for the connection pools (STRICT / LAX / OFFLOCK) driven by an injected test Clock.

Key points:

• No Thread.sleep() and no dependency on System.currentTimeMillis(): time is advanced explicitly via a test clock.
• Single-threaded lease operations use an immediate request timeout (0ms) to avoid waiting; pending is bounded via cancellation.
• Adds basic invariants that validate stats consistency and non-negative limits.