fix: improve model availability checks: Allows use of unavailable models on startup (#3717)

- Allows use of unavailable models on startup
- Add has_model method to ModelsRoutingTable for checking pre-registered
models
- Update check_model_availability to check model_store before provider
APIs

# What does this PR do?
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relevant issues if applicable. -->

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## Test Plan
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summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->


Start llama stack and point unavailable vLLM

```
VLLM_URL=https://my-unavailable-vllm/v1 MILVUS_DB_PATH=./milvus.db INFERENCE_MODEL=vllm uv run --with llama-stack llama stack build --distro starter --image-type venv --run
```

llama stack will start without crashing but only notifying error. 

```


         - provider_id: rag-runtime
           toolgroup_id: builtin::rag
         vector_dbs: []
         version: 2

INFO     2025-10-07 06:40:41,804 llama_stack.providers.utils.inference.inference_store:74 inference: Write queue disabled for SQLite to avoid concurrency issues
INFO     2025-10-07 06:40:42,066 llama_stack.providers.utils.responses.responses_store:96 openai_responses: Write queue disabled for SQLite to avoid concurrency issues
ERROR    2025-10-07 06:40:58,882 llama_stack.providers.utils.inference.openai_mixin:436 providers::utils: VLLMInferenceAdapter.list_provider_model_ids() failed with: Request timed out.
WARNING  2025-10-07 06:40:58,883 llama_stack.core.routing_tables.models:36 core::routing_tables: Model refresh failed for provider vllm: Request timed out.
[...]
INFO     2025-10-07 06:40:59,036 uvicorn.error:216 uncategorized: Uvicorn running on http://['::', '0.0.0.0']:8321 (Press CTRL+C to quit)
INFO     2025-10-07 06:41:04,064 openai._base_client:1618 uncategorized: Retrying request to /models in 0.398814 seconds
INFO     2025-10-07 06:41:09,497 openai._base_client:1618 uncategorized: Retrying request to /models in 0.781908 seconds
ERROR    2025-10-07 06:41:15,282 llama_stack.providers.utils.inference.openai_mixin:436 providers::utils: VLLMInferenceAdapter.list_provider_model_ids() failed with: Request timed out.
WARNING  2025-10-07 06:41:15,283 llama_stack.core.routing_tables.models:36 core::routing_tables: Model refresh failed for provider vllm: Request timed out.
```

tests/unit/distribution/routers/test_routing_tables.py

@@ -201,6 +201,12 @@ async def test_models_routing_table(cached_disk_dist_registry):
     non_existent = await table.get_object_by_identifier("model", "non-existent-model")
     assert non_existent is None
 
+    # Test has_model
+    assert await table.has_model("test_provider/test-model")
+    assert await table.has_model("test_provider/test-model-2")
+    assert not await table.has_model("non-existent-model")
+    assert not await table.has_model("test_provider/non-existent-model")
+
     await table.unregister_model(model_id="test_provider/test-model")
     await table.unregister_model(model_id="test_provider/test-model-2")