fix(engine): close AsyncAnthropic clients to prevent event loop shutdown crash (fixes #506)

[mickume]

Summary

Close all AsyncAnthropic clients after use via try/finally blocks. The Anthropic SDK wraps an httpx AsyncClient internally; when these clients were garbage-collected after the event loop closed, their __del__ scheduled aclose() tasks — producing two RuntimeError: Event loop is closed tracebacks on shutdown.

Closes #506

Changes

File	Change
agent_fox/core/client.py	Wrap ai_call()'s inner _call() in try/finally to close the client
agent_fox/knowledge/consolidation.py	Wrap _call_llm_json() body in try/finally to close the client
agent_fox/nightshift/categories/quality_gate.py	Close locally-created client (skip when backend is injected)
tests/unit/core/test_client_close.py	4 regression tests verifying clients are closed

Tests

• TestAiCallClosesClient::test_client_closed_on_success
• TestAiCallClosesClient::test_client_closed_on_api_error
• TestConsolidationCallLlmJsonClosesClient::test_client_closed_after_llm_call
• TestConsolidationCallLlmJsonClosesClient::test_client_closed_on_llm_error

Verification

• All existing tests pass: ✅ (5242 unchanged)
• New tests pass: ✅ (4 added)
• Linter / formatter: ✅
• No regressions: ✅

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