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/apis/resource.py

@@ -0,0 +1,37 @@
+# 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.
+
+
+from enum import StrEnum
+
+from pydantic import BaseModel, Field
+
+
+class ResourceType(StrEnum):
+    model = "model"
+    shield = "shield"
+    vector_store = "vector_store"
+    dataset = "dataset"
+    scoring_function = "scoring_function"
+    benchmark = "benchmark"
+    tool = "tool"
+    tool_group = "tool_group"
+    prompt = "prompt"
+
+
+class Resource(BaseModel):
+    """Base class for all Llama Stack resources"""
+
+    identifier: str = Field(description="Unique identifier for this resource in llama stack")
+
+    provider_resource_id: str | None = Field(
+        default=None,
+        description="Unique identifier for this resource in the provider",
+    )
+
+    provider_id: str = Field(description="ID of the provider that owns this resource")
+
+    type: ResourceType = Field(description="Type of resource (e.g. 'model', 'shield', 'vector_store', etc.)")