fix(deepseek): set default_aux_model on profile so aux warning stops firing

Closes #26924 (and supersedes #26926) in spirit.

DeepSeek was missing `default_aux_model` on its `ProviderProfile`, so
`_get_aux_model_for_provider("deepseek")` returned an empty string and
the compression / vision / session-search paths emitted

  "No auxiliary LLM provider configured -- context compression will
  drop middle turns without a summary."

on every DeepSeek session, even when the user had perfectly working
DeepSeek credentials.

Fix lands at the profile layer rather than the legacy
`_API_KEY_PROVIDER_AUX_MODELS_FALLBACK` dict the original PR targeted.
Every modern provider (gemini, zai, minimax, anthropic, kimi-coding,
stepfun, ollama-cloud, gmi, novita, kilocode, ai-gateway, opencode-zen)
sets `default_aux_model` on its `ProviderProfile`; the fallback dict
only exists for providers that predate the profiles system.

Tests added under `tests/plugins/model_providers/test_deepseek_profile.py`:
- `test_profile_advertises_deepseek_chat`  -- pins the profile attribute
- `test_consumer_api_returns_deepseek_chat` -- pins the consumer API behavior
- `test_consumer_api_returns_non_empty`     -- regression guard for the
  symptom in the issue

Original diagnosis and aux-model choice from @kriscolab in PR #26926;
moved one layer up.

Co-authored-by: kriscolab <71590782+kriscolab@users.noreply.github.com>

plugins/model-providers/deepseek/__init__.py

@@ -94,6 +94,7 @@ deepseek = DeepSeekProfile(
         "deepseek-reasoner",
     ),
     base_url="https://api.deepseek.com/v1",
+    default_aux_model="deepseek-chat",
 )
 
 register_provider(deepseek)