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The agents API is completely superseded by the Responses + Conversations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

tests/unit/providers/agents/meta_reference/fixtures/__init__.py

@@ -1,23 +0,0 @@
-# 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.
-
-import os
-
-import yaml
-
-from llama_stack.apis.inference import (
-    OpenAIChatCompletion,
-)
-
-FIXTURES_DIR = os.path.dirname(os.path.abspath(__file__))
-
-
-def load_chat_completion_fixture(filename: str) -> OpenAIChatCompletion:
-    fixture_path = os.path.join(FIXTURES_DIR, filename)
-
-    with open(fixture_path) as f:
-        data = yaml.safe_load(f)
-    return OpenAIChatCompletion(**data)

tests/unit/providers/agents/meta_reference/fixtures/simple_chat_completion.yaml

@@ -1,9 +0,0 @@
-id: chat-completion-123
-choices:
-  - message:
-      content: "Dublin"
-      role: assistant
-    finish_reason: stop
-    index: 0
-created: 1234567890
-model: meta-llama/Llama-3.1-8B-Instruct

tests/unit/providers/agents/meta_reference/fixtures/tool_call_completion.yaml

@@ -1,14 +0,0 @@
-id: chat-completion-123
-choices:
-  - message:
-      tool_calls:
-        - id: tool_call_123
-          type: function
-          function:
-            name: web_search
-            arguments: '{"query":"What is the capital of Ireland?"}'
-      role: assistant
-    finish_reason: stop
-    index: 0
-created: 1234567890
-model: meta-llama/Llama-3.1-8B-Instruct