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/vllm/config.py

@@ -0,0 +1,59 @@
+# 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 pathlib import Path
+
+from pydantic import Field, SecretStr, field_validator
+
+from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
+from llama_stack.schema_utils import json_schema_type
+
+
+@json_schema_type
+class VLLMInferenceAdapterConfig(RemoteInferenceProviderConfig):
+    url: str | None = Field(
+        default=None,
+        description="The URL for the vLLM model serving endpoint",
+    )
+    max_tokens: int = Field(
+        default=4096,
+        description="Maximum number of tokens to generate.",
+    )
+    auth_credential: SecretStr | None = Field(
+        default=None,
+        alias="api_token",
+        description="The API token",
+    )
+    tls_verify: bool | str = Field(
+        default=True,
+        description="Whether to verify TLS certificates. Can be a boolean or a path to a CA certificate file.",
+    )
+
+    @field_validator("tls_verify")
+    @classmethod
+    def validate_tls_verify(cls, v):
+        if isinstance(v, str):
+            # Otherwise, treat it as a cert path
+            cert_path = Path(v).expanduser().resolve()
+            if not cert_path.exists():
+                raise ValueError(f"TLS certificate file does not exist: {v}")
+            if not cert_path.is_file():
+                raise ValueError(f"TLS certificate path is not a file: {v}")
+            return v
+        return v
+
+    @classmethod
+    def sample_run_config(
+        cls,
+        url: str = "${env.VLLM_URL:=}",
+        **kwargs,
+    ):
+        return {
+            "url": url,
+            "max_tokens": "${env.VLLM_MAX_TOKENS:=4096}",
+            "api_token": "${env.VLLM_API_TOKEN:=fake}",
+            "tls_verify": "${env.VLLM_TLS_VERIFY:=true}",
+        }