feat(openai): add configurable base_url support with OPENAI_BASE_URL env var

- Add base_url field to OpenAIConfig with default "https://api.openai.com/v1"
- Update sample_run_config to support OPENAI_BASE_URL environment variable
- Modify get_base_url() to return configured base_url instead of hardcoded value
- Add comprehensive test suite covering:
  - Default base URL behavior
  - Custom base URL from config
  - Environment variable override
  - Config precedence over environment variables
  - Client initialization with configured URL
  - Model availability checks using configured URL

This enables users to configure custom OpenAI-compatible API endpoints
via environment variables or configuration files.

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

@@ -24,9 +24,19 @@ class OpenAIConfig(BaseModel):
         default=None,
         description="API key for OpenAI models",
     )
+    base_url: str = Field(
+        default="https://api.openai.com/v1",
+        description="Base URL for OpenAI API",
+    )
 
     @classmethod
-    def sample_run_config(cls, api_key: str = "${env.OPENAI_API_KEY:=}", **kwargs) -> dict[str, Any]:
+    def sample_run_config(
+        cls,
+        api_key: str = "${env.OPENAI_API_KEY:=}",
+        base_url: str = "${env.OPENAI_BASE_URL:=https://api.openai.com/v1}",
+        **kwargs,
+    ) -> dict[str, Any]:
         return {
             "api_key": api_key,
+            "base_url": base_url,
         }