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/gemini/gemini.py

@@ -0,0 +1,82 @@
+# 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 openai import NOT_GIVEN
+
+from llama_stack.apis.inference import (
+    OpenAIEmbeddingData,
+    OpenAIEmbeddingsRequestWithExtraBody,
+    OpenAIEmbeddingsResponse,
+    OpenAIEmbeddingUsage,
+)
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+from .config import GeminiConfig
+
+
+class GeminiInferenceAdapter(OpenAIMixin):
+    config: GeminiConfig
+
+    provider_data_api_key_field: str = "gemini_api_key"
+    embedding_model_metadata: dict[str, dict[str, int]] = {
+        "models/text-embedding-004": {"embedding_dimension": 768, "context_length": 2048},
+        "models/gemini-embedding-001": {"embedding_dimension": 3072, "context_length": 2048},
+    }
+
+    def get_base_url(self):
+        return "https://generativelanguage.googleapis.com/v1beta/openai/"
+
+    async def openai_embeddings(
+        self,
+        params: OpenAIEmbeddingsRequestWithExtraBody,
+    ) -> OpenAIEmbeddingsResponse:
+        """
+        Override embeddings method to handle Gemini's missing usage statistics.
+        Gemini's embedding API doesn't return usage information, so we provide default values.
+        """
+        # Prepare request parameters
+        request_params = {
+            "model": await self._get_provider_model_id(params.model),
+            "input": params.input,
+            "encoding_format": params.encoding_format if params.encoding_format is not None else NOT_GIVEN,
+            "dimensions": params.dimensions if params.dimensions is not None else NOT_GIVEN,
+            "user": params.user if params.user is not None else NOT_GIVEN,
+        }
+
+        # Add extra_body if present
+        extra_body = params.model_extra
+        if extra_body:
+            request_params["extra_body"] = extra_body
+
+        # Call OpenAI embeddings API with properly typed parameters
+        response = await self.client.embeddings.create(**request_params)
+
+        data = []
+        for i, embedding_data in enumerate(response.data):
+            data.append(
+                OpenAIEmbeddingData(
+                    embedding=embedding_data.embedding,
+                    index=i,
+                )
+            )
+
+        # Gemini doesn't return usage statistics - use default values
+        if hasattr(response, "usage") and response.usage:
+            usage = OpenAIEmbeddingUsage(
+                prompt_tokens=response.usage.prompt_tokens,
+                total_tokens=response.usage.total_tokens,
+            )
+        else:
+            usage = OpenAIEmbeddingUsage(
+                prompt_tokens=0,
+                total_tokens=0,
+            )
+
+        return OpenAIEmbeddingsResponse(
+            data=data,
+            model=params.model,
+            usage=usage,
+        )