feat: add /healthz and /metrics endpoints, replace print with logging

- Add /healthz endpoint returning LLM provider validation status
- Add /metrics endpoint serving Prometheus metrics via prometheus_client
- Replace all print() calls in health.py with proper logging module
- Remove _PREFIX variable in favor of structured logger context

pyproject.toml

@@ -2,10 +2,11 @@
 name = "pallas-mcp"
 version = "0.1.0"
 description = "FastAgent MCP Bridge — generic runtime for serving FastAgent agents over StreamableHTTP"
-requires-python = ">=3.13"
+requires-python = ">=3.13.5"
 dependencies = [
     "fast-agent-mcp>=0.6.10",
     "httpx",
+    "prometheus-client",
     "pyyaml",
     "starlette",
     "uvicorn",