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,
     )