feat: add OpenAI Responses API support via per-model api_flavor

Route opted-in models to /v1/responses while keeping Chat Completions as
the default. The flavor is selected per-model in the catalog YAML and
threaded through ModelCapabilities (with a new reasoning_effort field).

- Translate chat-shaped messages/tools/structured-output into the
  Responses input / text.format / flattened-tool shapes at the provider
  boundary; stored history and intra-turn tool round-trips need no
  migration.
- Normalize the typed SSE event stream back into the existing handler
  contract (content / thought / tool-call chunks) so the streaming
  accumulator and OpenAI handler stay unchanged.
- Carry encrypted reasoning items across the in-turn tool loop
  (provider-local, keyed by call_id) for stateless reasoning continuity.
- OPENAI_RESPONSES_STORE opt-in enables server-side state and
  previous_response_id chaining across turns, persisted in message
  metadata.
- Set gpt-5.5 to api_flavor: responses, reasoning_effort: medium.
- Remove AzureOpenAILLM and the azure_openai provider/catalog.

tests/core/test_model_utils.py

@@ -156,16 +156,6 @@ class TestGetApiKeyForProvider:
 
             assert get_api_key_for_provider("unknown_provider") == "sk-fallback"
 
-    @pytest.mark.unit
-    def test_azure_openai_key(self):
-        with patch("application.core.settings.settings") as mock_settings:
-            mock_settings.API_KEY = "sk-azure"
-
-            from application.core.model_utils import get_api_key_for_provider
-
-            assert get_api_key_for_provider("azure_openai") == "sk-azure"
-
-
 # ── get_all_available_models ─────────────────────────────────────────────────