Enforces config env var checks and improves startup handling (#892)

* Enforces config env var checks and improves startup handling

Ensures critical environment variables are validated during config resolution,
raising clear errors if missing. Improves server startup reliability by
verifying that backend services are listening and by terminating on
misconfiguration at launch. Adds more robust feedback to developers when
API startup fails, reducing silent misconfigurations and speeding up
troubleshooting.

* Initial plan

* Implement suggestions from PR #892: fix env var checks and improve error logging

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

Makefile

@@ -187,7 +187,12 @@ dev:
 	echo "✓ LangGraph server started on localhost:2024"; \
 	echo "Starting Gateway API..."; \
 	cd backend && uv run uvicorn src.gateway.app:app --host 0.0.0.0 --port 8001 > ../logs/gateway.log 2>&1 & \
-	sleep 2; \
+	sleep 3; \
+	if ! lsof -i :8001 -sTCP:LISTEN -t >/dev/null 2>&1; then \
+		echo "✗ Gateway API failed to start. Last log output:"; \
+		tail -30 logs/gateway.log; \
+		cleanup; \
+	fi; \
 	echo "✓ Gateway API started on localhost:8001"; \
 	echo "Starting Frontend..."; \
 	cd frontend && pnpm run dev > ../logs/frontend.log 2>&1 & \