feat: use SecretStr for inference provider auth credentials (#3724)

# What does this PR do?

use SecretStr for OpenAIMixin providers

- RemoteInferenceProviderConfig now has auth_credential: SecretStr
- the default alias is api_key (most common name)
- some providers override to use api_token (RunPod, vLLM, Databricks)
- some providers exclude it (Ollama, TGI, Vertex AI)

addresses #3517 

## Test Plan

ci w/ new tests

llama_stack/providers/remote/inference/anthropic/anthropic.py

@@ -29,9 +29,6 @@ class AnthropicInferenceAdapter(OpenAIMixin):
     #     "voyage-multimodal-3": {"embedding_dimension": 1024, "context_length": 32000},
     # }
 
-    def get_api_key(self) -> str:
-        return self.config.api_key or ""
-
     def get_base_url(self):
         return "https://api.anthropic.com/v1"
 

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

@@ -21,11 +21,6 @@ class AnthropicProviderDataValidator(BaseModel):
 
 @json_schema_type
 class AnthropicConfig(RemoteInferenceProviderConfig):
-    api_key: str | None = Field(
-        default=None,
-        description="API key for Anthropic models",
-    )
-
     @classmethod
     def sample_run_config(cls, api_key: str = "${env.ANTHROPIC_API_KEY:=}", **kwargs) -> dict[str, Any]:
         return {