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/safety/safety.py

@@ -0,0 +1,135 @@
+# 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 Enum
+from typing import Any, Protocol, runtime_checkable
+
+from pydantic import BaseModel, Field
+
+from llama_stack.apis.inference import OpenAIMessageParam
+from llama_stack.apis.shields import Shield
+from llama_stack.apis.version import LLAMA_STACK_API_V1
+from llama_stack.core.telemetry.trace_protocol import trace_protocol
+from llama_stack.schema_utils import json_schema_type, webmethod
+
+
+@json_schema_type
+class ModerationObjectResults(BaseModel):
+    """A moderation object.
+    :param flagged: Whether any of the below categories are flagged.
+    :param categories: A list of the categories, and whether they are flagged or not.
+    :param category_applied_input_types: A list of the categories along with the input type(s) that the score applies to.
+    :param category_scores: A list of the categories along with their scores as predicted by model.
+    """
+
+    flagged: bool
+    categories: dict[str, bool] | None = None
+    category_applied_input_types: dict[str, list[str]] | None = None
+    category_scores: dict[str, float] | None = None
+    user_message: str | None = None
+    metadata: dict[str, Any] = Field(default_factory=dict)
+
+
+@json_schema_type
+class ModerationObject(BaseModel):
+    """A moderation object.
+    :param id: The unique identifier for the moderation request.
+    :param model: The model used to generate the moderation results.
+    :param results: A list of moderation objects
+    """
+
+    id: str
+    model: str
+    results: list[ModerationObjectResults]
+
+
+@json_schema_type
+class ViolationLevel(Enum):
+    """Severity level of a safety violation.
+
+    :cvar INFO: Informational level violation that does not require action
+    :cvar WARN: Warning level violation that suggests caution but allows continuation
+    :cvar ERROR: Error level violation that requires blocking or intervention
+    """
+
+    INFO = "info"
+    WARN = "warn"
+    ERROR = "error"
+
+
+@json_schema_type
+class SafetyViolation(BaseModel):
+    """Details of a safety violation detected by content moderation.
+
+    :param violation_level: Severity level of the violation
+    :param user_message: (Optional) Message to convey to the user about the violation
+    :param metadata: Additional metadata including specific violation codes for debugging and telemetry
+    """
+
+    violation_level: ViolationLevel
+
+    # what message should you convey to the user
+    user_message: str | None = None
+
+    # additional metadata (including specific violation codes) more for
+    # debugging, telemetry
+    metadata: dict[str, Any] = Field(default_factory=dict)
+
+
+@json_schema_type
+class RunShieldResponse(BaseModel):
+    """Response from running a safety shield.
+
+    :param violation: (Optional) Safety violation detected by the shield, if any
+    """
+
+    violation: SafetyViolation | None = None
+
+
+class ShieldStore(Protocol):
+    async def get_shield(self, identifier: str) -> Shield: ...
+
+
+@runtime_checkable
+@trace_protocol
+class Safety(Protocol):
+    """Safety
+
+    OpenAI-compatible Moderations API.
+    """
+
+    shield_store: ShieldStore
+
+    @webmethod(route="/safety/run-shield", method="POST", level=LLAMA_STACK_API_V1)
+    async def run_shield(
+        self,
+        shield_id: str,
+        messages: list[OpenAIMessageParam],
+        params: dict[str, Any],
+    ) -> RunShieldResponse:
+        """Run shield.
+
+        Run a shield.
+
+        :param shield_id: The identifier of the shield to run.
+        :param messages: The messages to run the shield on.
+        :param params: The parameters of the shield.
+        :returns: A RunShieldResponse.
+        """
+        ...
+
+    @webmethod(route="/openai/v1/moderations", method="POST", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/moderations", method="POST", level=LLAMA_STACK_API_V1)
+    async def run_moderation(self, input: str | list[str], model: str | None = None) -> ModerationObject:
+        """Create moderation.
+
+        Classifies if text and/or image inputs are potentially harmful.
+        :param input: Input (or inputs) to classify.
+        Can be a single string, an array of strings, or an array of multi-modal input objects similar to other models.
+        :param model: (Optional) The content moderation model you would like to use.
+        :returns: A moderation object.
+        """
+        ...