config: rename the DeepSeek flash model to deepseek-flash

DeepSeek dropped the `v4` prefix from its flash model, and the old id stays
valid only as a compatibility alias. Seed the default catalog with the current
name so a provider that omits `models` no longer offers a deprecated id, and
follow the rename in the wire fixtures and the minimal config template. An
explicit `deepseek-v4-flash` in TOML still passes through unchanged.

tests/test_agent/test_responses_bridge.py

@@ -167,7 +167,7 @@ def _reasoning_response(
     return Response(
         id="resp_r",
         created_at=1,
-        model="deepseek-v4-flash",
+        model="deepseek-flash",
         output=output,
         reasoning=top_level,
     )

tests/test_agent/test_responses_dispatch.py

@@ -430,7 +430,7 @@ async def test_dispatch_stream_failed_raises_with_error_detail() -> None:
                 "id": "resp_1",
                 "object": "response",
                 "created_at": 1,
-                "model": "deepseek-v4-flash",
+                "model": "deepseek-flash",
                 "status": "failed",
                 "output": [],
                 "error": {"code": "rate_limit_exceeded", "message": "slow down"},