fix: add logging to empty exception handlers in _base_client.py

[Sahith59]

Fixes #3428

Found an empty exception handler except: pass pattern in _base_client.py. Empty exception handlers silently swallow errors, which can make debugging very difficult.

This PR replaces pass with a debug log message when closing the client connections.

[jbeckwith-oai]

Blocking: these handlers are inside __del__ finalizers, not ordinary operational code, and the swallowed exceptions include expected teardown control flow. In particular, AsyncHttpxClientWrapper.__del__ calls asyncio.get_running_loop(), which normally raises when GC/interpreter shutdown runs outside an active event loop. This change turns that routine path into a full traceback whenever SDK debug logging is enabled. Reproduction on this head:

OPENAI_LOG=debug PYTHONPATH=src python -c "from openai import AsyncOpenAI; AsyncOpenAI(api_key=\"test\")"

It emits Failed to close async client plus RuntimeError: no running event loop during otherwise successful process exit. Logging from a finalizer is also not teardown-safe, and exc_info=True introduces a new disclosure surface because SensitiveHeadersFilter only sanitizes structured header arguments—not arbitrary transport/proxy exception text and tracebacks.

Please keep this best-effort fallback silent and document why, or at minimum distinguish the expected no-loop/finalizing cases and emit only a sanitized message when logging is known to be safe. Add focused lifecycle tests for normal GC outside a loop, an actual close failure, and interpreter teardown. Explicit close() / aclose() remains the reliable path where callers can observe cleanup failures.

Validation on exact head: Ruff passed and tests/test_client.py passed (198 passed, 2 skipped); the problem is the newly introduced finalizer/logging behavior, not a syntax or test regression.