chore!: remove the agents (sessions and turns) API (#4055)

- Removes the deprecated agents (sessions and turns) API that was marked
alpha in 0.3.0
- Cleans up unused imports and orphaned types after the API removal
- Removes `SessionNotFoundError` and `AgentTurnInputType` which are no
longer needed

The agents API is completely superseded by the Responses + Conversations
APIs, and the client SDK Agent class already uses those implementations.

Corresponding client-side PR:
https://github.com/llamastack/llama-stack-client-python/pull/295

src/llama_stack/apis/common/errors.py

@@ -56,14 +56,6 @@ class ToolGroupNotFoundError(ResourceNotFoundError):
         super().__init__(toolgroup_name, "Tool Group", "client.toolgroups.list()")
 
 
-class SessionNotFoundError(ValueError):
-    """raised when Llama Stack cannot find a referenced session or access is denied"""
-
-    def __init__(self, session_name: str) -> None:
-        message = f"Session '{session_name}' not found or access denied."
-        super().__init__(message)
-
-
 class ModelTypeError(TypeError):
     """raised when a model is present but not the correct type"""
 

src/llama_stack/apis/common/type_system.py

@@ -103,17 +103,6 @@ class CompletionInputType(BaseModel):
     type: Literal["completion_input"] = "completion_input"
 
 
-@json_schema_type
-class AgentTurnInputType(BaseModel):
-    """Parameter type for agent turn input.
-
-    :param type: Discriminator type. Always "agent_turn_input"
-    """
-
-    # expects List[Message] for messages (may also include attachments?)
-    type: Literal["agent_turn_input"] = "agent_turn_input"
-
-
 @json_schema_type
 class DialogType(BaseModel):
     """Parameter type for dialog data with semantic output labels.
@@ -135,8 +124,7 @@ ParamType = Annotated[
     | JsonType
     | UnionType
     | ChatCompletionInputType
-    | CompletionInputType
-    | AgentTurnInputType,
+    | CompletionInputType,
     Field(discriminator="type"),
 ]
 register_schema(ParamType, name="ParamType")