feat: Add clear error message when API key is missing (#2992)

# What does this PR do?

Improve user experience by providing specific guidance when no API key
is available, showing both provider data header and config options with
the correct field name for each provider.

Also adds comprehensive test coverage for API key resolution scenarios.

addresses #2990 for providers using litellm openai mixin

## Test Plan

`./scripts/unit-tests.sh
tests/unit/providers/inference/test_litellm_openai_mixin.py`

llama_stack/providers/utils/inference/litellm_openai_mixin.py

@@ -254,6 +254,12 @@ class LiteLLMOpenAIMixin(
             api_key = getattr(provider_data, key_field)
         else:
             api_key = self.api_key_from_config
+        if not api_key:
+            raise ValueError(
+                "API key is not set. Please provide a valid API key in the "
+                "provider data header, e.g. x-llamastack-provider-data: "
+                f'{{"{key_field}": "<API_KEY>"}}, or in the provider config.'
+            )
         return api_key
 
     async def embeddings(

tests/unit/providers/inference/test_litellm_openai_mixin.py

@@ -0,0 +1,112 @@
+# 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 json
+from unittest.mock import MagicMock
+
+import pytest
+from pydantic import BaseModel, Field
+
+from llama_stack.core.request_headers import request_provider_data_context
+from llama_stack.providers.utils.inference.litellm_openai_mixin import LiteLLMOpenAIMixin
+
+
+# Test fixtures and helper classes
+class TestConfig(BaseModel):
+    api_key: str | None = Field(default=None)
+
+
+class TestProviderDataValidator(BaseModel):
+    test_api_key: str | None = Field(default=None)
+
+
+class TestLiteLLMAdapter(LiteLLMOpenAIMixin):
+    def __init__(self, config: TestConfig):
+        super().__init__(
+            model_entries=[],
+            litellm_provider_name="test",
+            api_key_from_config=config.api_key,
+            provider_data_api_key_field="test_api_key",
+            openai_compat_api_base=None,
+        )
+
+
+@pytest.fixture
+def adapter_with_config_key():
+    """Fixture to create adapter with API key in config"""
+    config = TestConfig(api_key="config-api-key")
+    adapter = TestLiteLLMAdapter(config)
+    adapter.__provider_spec__ = MagicMock()
+    adapter.__provider_spec__.provider_data_validator = (
+        "tests.unit.providers.inference.test_litellm_openai_mixin.TestProviderDataValidator"
+    )
+    return adapter
+
+
+@pytest.fixture
+def adapter_without_config_key():
+    """Fixture to create adapter without API key in config"""
+    config = TestConfig(api_key=None)
+    adapter = TestLiteLLMAdapter(config)
+    adapter.__provider_spec__ = MagicMock()
+    adapter.__provider_spec__.provider_data_validator = (
+        "tests.unit.providers.inference.test_litellm_openai_mixin.TestProviderDataValidator"
+    )
+    return adapter
+
+
+def test_api_key_from_config_when_no_provider_data(adapter_with_config_key):
+    """Test that adapter uses config API key when no provider data is available"""
+    api_key = adapter_with_config_key.get_api_key()
+    assert api_key == "config-api-key"
+
+
+def test_provider_data_takes_priority_over_config(adapter_with_config_key):
+    """Test that provider data API key overrides config API key"""
+    with request_provider_data_context(
+        {"x-llamastack-provider-data": json.dumps({"test_api_key": "provider-data-key"})}
+    ):
+        api_key = adapter_with_config_key.get_api_key()
+        assert api_key == "provider-data-key"
+
+
+def test_fallback_to_config_when_provider_data_missing_key(adapter_with_config_key):
+    """Test fallback to config when provider data doesn't have the required key"""
+    with request_provider_data_context({"x-llamastack-provider-data": json.dumps({"wrong_key": "some-value"})}):
+        api_key = adapter_with_config_key.get_api_key()
+        assert api_key == "config-api-key"
+
+
+def test_error_when_no_api_key_available(adapter_without_config_key):
+    """Test that ValueError is raised when neither config nor provider data have API key"""
+    with pytest.raises(ValueError, match="API key is not set"):
+        adapter_without_config_key.get_api_key()
+
+
+def test_error_when_provider_data_has_wrong_key(adapter_without_config_key):
+    """Test that ValueError is raised when provider data exists but doesn't have required key"""
+    with request_provider_data_context({"x-llamastack-provider-data": json.dumps({"wrong_key": "some-value"})}):
+        with pytest.raises(ValueError, match="API key is not set"):
+            adapter_without_config_key.get_api_key()
+
+
+def test_provider_data_works_when_config_is_none(adapter_without_config_key):
+    """Test that provider data works even when config has no API key"""
+    with request_provider_data_context(
+        {"x-llamastack-provider-data": json.dumps({"test_api_key": "provider-only-key"})}
+    ):
+        api_key = adapter_without_config_key.get_api_key()
+        assert api_key == "provider-only-key"
+
+
+def test_error_message_includes_correct_field_names(adapter_without_config_key):
+    """Test that error message includes correct field name and header information"""
+    try:
+        adapter_without_config_key.get_api_key()
+        raise AssertionError("Should have raised ValueError")
+    except ValueError as e:
+        assert "test_api_key" in str(e)  # Should mention the correct field name
+        assert "x-llamastack-provider-data" in str(e)  # Should mention header name