feat: add refresh_models support to inference adapters (default: false) (#3719)

# What does this PR do?

inference adapters can now configure `refresh_models: bool` to control
periodic model listing from their providers

BREAKING CHANGE: together inference adapter default changed. previously
always refreshed, now follows config.

addresses "models: refresh" on #3517

## Test Plan

ci w/ new tests

llama_stack/providers/remote/inference/vllm/config.py

@@ -30,10 +30,6 @@ class VLLMInferenceAdapterConfig(RemoteInferenceProviderConfig):
         default=True,
         description="Whether to verify TLS certificates. Can be a boolean or a path to a CA certificate file.",
     )
-    refresh_models: bool = Field(
-        default=False,
-        description="Whether to refresh models periodically",
-    )
 
     @field_validator("tls_verify")
     @classmethod

llama_stack/providers/remote/inference/vllm/vllm.py

@@ -53,10 +53,6 @@ class VLLMInferenceAdapter(OpenAIMixin):
                 "You must provide a URL in run.yaml (or via the VLLM_URL environment variable) to use vLLM."
             )
 
-    async def should_refresh_models(self) -> bool:
-        # Strictly respecting the refresh_models directive
-        return self.config.refresh_models
-
     async def health(self) -> HealthResponse:
         """
         Performs a health check by verifying connectivity to the remote vLLM server.