fix(server): avoid reference leaks on rejected session deletion - #1803

Summary

• Check a session's cache reference count before retrieving its instance, so a rejected deletion does not retain an extra reference.
• Cover zero-capacity and bounded caches, including a successful retry after the active request releases its reference.

Validation

• Focused manager tests: uv run --frozen --all-extras pytest packages/server/server_tests/memmachine_server/episodic_memory/test_episodic_memory_manager.py (15 passed).
• Client tests: uv run --frozen --all-extras pytest packages/client/client_tests --cov=memmachine_client --cov-append (258 passed, 79 deselected).
• Ruff lint/format, uv lock --check, and type checks for common/client (Python 3.10 and 3.14) and server (Python 3.12 and 3.14) passed.
• Full server suite: 2,099 passed, 3 skipped, 3 failed due to OpenAI API request timeouts; the same timeouts were reproduced on the clean base.

Fixes #1765

Issue: #1765