Serialize shared Aerospike client calls

[ZardLi1115]

Summary

• Add a per-process call gate keyed by Aerospike client identity so concurrent sessions sharing one synchronous client do not enter the C client concurrently.
• Route AerospikeSession and ShardedAerospikeSession blocking client work through the shared gate.
• Add a regression test with a blocking fake client to prove shared-client calls are serialized across sessions.

Tests

• ruff format --check src/openai_agents_aerospike/session.py src/openai_agents_aerospike/sharded_session.py tests/test_session.py
• ruff check .
• mypy src/openai_agents_aerospike
• pytest -q tests/test_session.py::test_shared_client_calls_are_serialized_across_sessions
• pytest -q tests/test_import.py tests/test_session.py (Aerospike integration tests skipped because AEROSPIKE_HOST is unset)
• pytest -q (Aerospike integration tests skipped because AEROSPIKE_HOST is unset)

Note: ruff format --check . still reports pre-existing formatting drift in benchmarks/infra/orchestrator/summarize_sweep.py, which this PR does not touch.

Fixes #13