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/cerebras/cerebras.py

@@ -0,0 +1,28 @@
+# 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 urllib.parse import urljoin
+
+from llama_stack.apis.inference import (
+    OpenAIEmbeddingsRequestWithExtraBody,
+    OpenAIEmbeddingsResponse,
+)
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+from .config import CerebrasImplConfig
+
+
+class CerebrasInferenceAdapter(OpenAIMixin):
+    config: CerebrasImplConfig
+
+    def get_base_url(self) -> str:
+        return urljoin(self.config.base_url, "v1")
+
+    async def openai_embeddings(
+        self,
+        params: OpenAIEmbeddingsRequestWithExtraBody,
+    ) -> OpenAIEmbeddingsResponse:
+        raise NotImplementedError()