Fix router disabled provider check + comprehensive functional tests

Fixes:
- Router now properly skips disabled providers in complete() method
- Fixed avg_latency calculation comment in tests (now correctly documents behavior)

New Test Suites:
- tests/test_functional_router.py: 10 functional tests for router
- tests/test_functional_mcp.py: 15 functional tests for MCP discovery/bootstrap
- tests/test_integration_full.py: 14 end-to-end integration tests

Total: 39 new functional/integration tests

All 144 tests passing (105 router/mcp + 39 functional/integration)

src/router/cascade.py

@@ -250,6 +250,11 @@ class CascadeRouter:
         errors = []
         
         for provider in self.providers:
+            # Skip disabled providers
+            if not provider.enabled:
+                logger.debug("Skipping %s (disabled)", provider.name)
+                continue
+            
             # Skip unhealthy providers (circuit breaker)
             if provider.status == ProviderStatus.UNHEALTHY:
                 # Check if circuit breaker can close