chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/providers/remote/inference/vertexai/vertexai.py

@@ -0,0 +1,44 @@
+# 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 google.auth.transport.requests
+from google.auth import default
+
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+from .config import VertexAIConfig
+
+
+class VertexAIInferenceAdapter(OpenAIMixin):
+    config: VertexAIConfig
+
+    provider_data_api_key_field: str = "vertex_project"
+
+    def get_api_key(self) -> str:
+        """
+        Get an access token for Vertex AI using Application Default Credentials.
+
+        Vertex AI uses ADC instead of API keys. This method obtains an access token
+        from the default credentials and returns it for use with the OpenAI-compatible client.
+        """
+        try:
+            # Get default credentials - will read from GOOGLE_APPLICATION_CREDENTIALS
+            credentials, _ = default(scopes=["https://www.googleapis.com/auth/cloud-platform"])
+            credentials.refresh(google.auth.transport.requests.Request())
+            return str(credentials.token)
+        except Exception:
+            # If we can't get credentials, return empty string to let the env work with ADC directly
+            return ""
+
+    def get_base_url(self) -> str:
+        """
+        Get the Vertex AI OpenAI-compatible API base URL.
+
+        Returns the Vertex AI OpenAI-compatible endpoint URL.
+        Source: https://cloud.google.com/vertex-ai/generative-ai/docs/start/openai
+        """
+        return f"https://{self.config.location}-aiplatform.googleapis.com/v1/projects/{self.config.project}/locations/{self.config.location}/endpoints/openapi"