feat(openai): add configurable base_url support with OPENAI_BASE_URL env var (#2919)

# What does this PR do?

- 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.

Closes #2910 

## Test Plan

run unit tests

tests/unit/providers/inference/test_openai_base_url_config.py

@@ -0,0 +1,125 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import os
+from unittest.mock import AsyncMock, MagicMock, patch
+
+from llama_stack.distribution.stack import replace_env_vars
+from llama_stack.providers.remote.inference.openai.config import OpenAIConfig
+from llama_stack.providers.remote.inference.openai.openai import OpenAIInferenceAdapter
+
+
+class TestOpenAIBaseURLConfig:
+    """Test that OPENAI_BASE_URL environment variable properly configures the OpenAI adapter."""
+
+    def test_default_base_url_without_env_var(self):
+        """Test that the adapter uses the default OpenAI base URL when no environment variable is set."""
+        config = OpenAIConfig(api_key="test-key")
+        adapter = OpenAIInferenceAdapter(config)
+
+        assert adapter.get_base_url() == "https://api.openai.com/v1"
+
+    def test_custom_base_url_from_config(self):
+        """Test that the adapter uses a custom base URL when provided in config."""
+        custom_url = "https://custom.openai.com/v1"
+        config = OpenAIConfig(api_key="test-key", base_url=custom_url)
+        adapter = OpenAIInferenceAdapter(config)
+
+        assert adapter.get_base_url() == custom_url
+
+    @patch.dict(os.environ, {"OPENAI_BASE_URL": "https://env.openai.com/v1"})
+    def test_base_url_from_environment_variable(self):
+        """Test that the adapter uses base URL from OPENAI_BASE_URL environment variable."""
+        # Use sample_run_config which has proper environment variable syntax
+        config_data = OpenAIConfig.sample_run_config(api_key="test-key")
+        processed_config = replace_env_vars(config_data)
+        config = OpenAIConfig.model_validate(processed_config)
+        adapter = OpenAIInferenceAdapter(config)
+
+        assert adapter.get_base_url() == "https://env.openai.com/v1"
+
+    @patch.dict(os.environ, {"OPENAI_BASE_URL": "https://env.openai.com/v1"})
+    def test_config_overrides_environment_variable(self):
+        """Test that explicit config value overrides environment variable."""
+        custom_url = "https://config.openai.com/v1"
+        config = OpenAIConfig(api_key="test-key", base_url=custom_url)
+        adapter = OpenAIInferenceAdapter(config)
+
+        # Config should take precedence over environment variable
+        assert adapter.get_base_url() == custom_url
+
+    @patch("llama_stack.providers.utils.inference.openai_mixin.AsyncOpenAI")
+    def test_client_uses_configured_base_url(self, mock_openai_class):
+        """Test that the OpenAI client is initialized with the configured base URL."""
+        custom_url = "https://test.openai.com/v1"
+        config = OpenAIConfig(api_key="test-key", base_url=custom_url)
+        adapter = OpenAIInferenceAdapter(config)
+
+        # Mock the get_api_key method since it's delegated to LiteLLMOpenAIMixin
+        adapter.get_api_key = MagicMock(return_value="test-key")
+
+        # Access the client property to trigger AsyncOpenAI initialization
+        _ = adapter.client
+
+        # Verify AsyncOpenAI was called with the correct base_url
+        mock_openai_class.assert_called_once_with(
+            api_key="test-key",
+            base_url=custom_url,
+        )
+
+    @patch("llama_stack.providers.utils.inference.openai_mixin.AsyncOpenAI")
+    async def test_check_model_availability_uses_configured_url(self, mock_openai_class):
+        """Test that check_model_availability uses the configured base URL."""
+        custom_url = "https://test.openai.com/v1"
+        config = OpenAIConfig(api_key="test-key", base_url=custom_url)
+        adapter = OpenAIInferenceAdapter(config)
+
+        # Mock the get_api_key method
+        adapter.get_api_key = MagicMock(return_value="test-key")
+
+        # Mock the AsyncOpenAI client and its models.retrieve method
+        mock_client = MagicMock()
+        mock_client.models.retrieve = AsyncMock(return_value=MagicMock())
+        mock_openai_class.return_value = mock_client
+
+        # Call check_model_availability and verify it returns True
+        assert await adapter.check_model_availability("gpt-4")
+
+        # Verify the client was created with the custom URL
+        mock_openai_class.assert_called_with(
+            api_key="test-key",
+            base_url=custom_url,
+        )
+
+        # Verify the method was called and returned True
+        mock_client.models.retrieve.assert_called_once_with("gpt-4")
+
+    @patch.dict(os.environ, {"OPENAI_BASE_URL": "https://proxy.openai.com/v1"})
+    @patch("llama_stack.providers.utils.inference.openai_mixin.AsyncOpenAI")
+    async def test_environment_variable_affects_model_availability_check(self, mock_openai_class):
+        """Test that setting OPENAI_BASE_URL environment variable affects where model availability is checked."""
+        # Use sample_run_config which has proper environment variable syntax
+        config_data = OpenAIConfig.sample_run_config(api_key="test-key")
+        processed_config = replace_env_vars(config_data)
+        config = OpenAIConfig.model_validate(processed_config)
+        adapter = OpenAIInferenceAdapter(config)
+
+        # Mock the get_api_key method
+        adapter.get_api_key = MagicMock(return_value="test-key")
+
+        # Mock the AsyncOpenAI client
+        mock_client = MagicMock()
+        mock_client.models.retrieve = AsyncMock(return_value=MagicMock())
+        mock_openai_class.return_value = mock_client
+
+        # Call check_model_availability and verify it returns True
+        assert await adapter.check_model_availability("gpt-4")
+
+        # Verify the client was created with the environment variable URL
+        mock_openai_class.assert_called_with(
+            api_key="test-key",
+            base_url="https://proxy.openai.com/v1",
+        )