test: assert Logger output for backpressure and thread-pool worker exceptions

[mvillmow]

Summary

• Adds AgentCoreLogTest suite to test_agent_core.cpp with two new tests:
  • BackpressureInboxFullLogsWarn — verifies [BACKPRESSURE] ... inbox full is logged at warn level when AgentCore receives messages beyond AGENT_MAX_QUEUE_SIZE
  • BackpressureRecoveryLogsInfo — verifies ... recovered ... accepting messages is logged at info level once the queue drains below the low watermark
• Adds ThreadPoolLogTest suite to test_thread_pool.cpp with two new tests:
  • WorkerStdExceptionIsLogged — verifies ThreadPool: Exception in worker: <msg> is logged at error when a task throws std::exception
  • WorkerUnknownExceptionIsLogged — verifies ThreadPool: Unknown exception in worker is logged at error for non-std::exception throws
• Uses spdlog::sinks::ringbuffer_sink_mt injected via spdlog::get("keystone")->sinks() so assertions run against real spdlog output without requiring stderr capture or mocking.

Closes #337

Test plan

• cmake --preset debug && cmake --build --preset debug && ctest --preset debug -R "AgentCoreLog|ThreadPoolLog" --output-on-failure
• cmake --preset asan && cmake --build --preset asan && ctest --preset asan -R "AgentCoreLog|ThreadPoolLog" --output-on-failure
• just format-check passes (verified locally)

🤖 Generated with Claude Code

[github-actions]

✅ Dependency Audit

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See the Security tab for detailed findings.

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Security Scan Results

• ❌ Secret Scanning: Potential secrets found
• ✅ SAST: Completed (check Security tab for details)
• ✅ Dependency Scanning: Completed
• ✅ C++ Static Analysis: Completed
• ✅ Docker Image Scanning: 0 high, 22 medium vulnerabilities (acceptable)

Recommendations

• Review findings in the GitHub Security tab
• Check artifact uploads for detailed reports
• Address critical Docker vulnerabilities immediately

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Workflow: Security Scanning