[ Bounty $4k ] [ Metrics ] Reject non-finite gauge values - numeric safety

[MyTH-zyxeon]

Summary

• reject NaN and infinite gauge values before they reach snapshots/exporters
• add regression coverage for NaN, +inf, and -inf without leaving rejected values in gauge state
• unblock full local pytest by exporting AgentStatus and using an RLock for timer observations

Root cause

MetricsCollector.gauge() trusted callers and stored non-finite floats directly. Those values then appeared in snapshot(), which can break JSON-compatible exporters or cause them to drop a metrics batch.

Validation

• ./.venv/bin/python -m pytest tests/test_metrics.py -q
• ./.venv/bin/python -m pytest tests/test_metrics.py tests/test_agent_registry.py -q
• ./.venv/bin/python -m pytest -q
• ./.venv/bin/python -m flake8 src/common/metrics.py src/agent/__init__.py tests/test_metrics.py
• ./.venv/bin/python -m compileall -q src tests
• git diff --check
• gh repo view orchestration-agent/AgentOrchestration --json viewerHasStarred --jq .viewerHasStarred -> true

Bounty issue: #3873