fix(routing): union configured and known providers

[coniferconcepts]

Summary

• treat a provider as valid for routing override validation when it appears in either the configured provider set or the loaded plugin set
• fix a standard-launcher startup edge case where configured providers like ollama could be rejected as unknown when known_providers came from a narrower plugin list
• add a focused regression test covering the exact available-vs-known mismatch that the local runtime exposed after the v1/v2 routing work merged

Testing

• pytest tests/test_routing_policy.py tests/test_client_routing_policy.py
• python3 -m py_compile "src/rotator_library/routing_policy.py" "tests/test_routing_policy.py"

Guardrails

• no routing selection semantics change beyond provider validation
• weighted and single routing behavior remain unchanged after validation succeeds
• this only widens provider recognition to the safe union of configured providers and loaded providers

Runtime Context

• this follow-up came from a real local ./scripts/start-all.sh failure
• the local runtime now starts cleanly with the merged v1/v2 overrides after this fix