Python: lazy-loading mechanism for agent dependencies (#11180)

### Motivation and Context

Consolidated agent imports initially caused issues by eagerly loading
all submodules, which forced users to install every optional dependency
-- even if only one agent was needed. This PR introduces a lazy-loading
mechanism via `__getattr__` in the top-level `__init__.py` to defer
imports until actually accessed. Additionally, a corresponding`
__init__.pyi` type stub file is provided to restore full IDE
autocompletion and type hints without compromising runtime behavior.

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### Description

Lazy load agent dependencies.

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These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: moonbox3

python/semantic_kernel/__init__.py

@@ -2,8 +2,8 @@
 
 from semantic_kernel.kernel import Kernel
 
-__version__ = "1.26.0"
+__version__ = "1.26.1"
 
-DEFAULT_RC_VERSION = f"{__version__}-rc4"
+DEFAULT_RC_VERSION = f"{__version__}-rc5"
 
 __all__ = ["DEFAULT_RC_VERSION", "Kernel", "__version__"]

python/semantic_kernel/agents/__init__.py

@@ -1,35 +1,35 @@
 # Copyright (c) Microsoft. All rights reserved.
 
-from semantic_kernel.agents.agent import Agent, AgentResponseItem, AgentThread
-from semantic_kernel.agents.autogen.autogen_conversable_agent import (
-    AutoGenConversableAgent,
-    AutoGenConversableAgentThread,
-)
-from semantic_kernel.agents.azure_ai.azure_ai_agent import AzureAIAgent, AzureAIAgentThread
-from semantic_kernel.agents.azure_ai.azure_ai_agent_settings import AzureAIAgentSettings
-from semantic_kernel.agents.bedrock.bedrock_agent import BedrockAgent, BedrockAgentThread
-from semantic_kernel.agents.chat_completion.chat_completion_agent import ChatCompletionAgent, ChatHistoryAgentThread
-from semantic_kernel.agents.group_chat.agent_chat import AgentChat
-from semantic_kernel.agents.group_chat.agent_group_chat import AgentGroupChat
-from semantic_kernel.agents.open_ai.azure_assistant_agent import AzureAssistantAgent
-from semantic_kernel.agents.open_ai.open_ai_assistant_agent import AssistantAgentThread, OpenAIAssistantAgent
+import importlib
 
-__all__ = [
-    "Agent",
-    "AgentChat",
-    "AgentGroupChat",
-    "AgentResponseItem",
-    "AgentThread",
-    "AssistantAgentThread",
-    "AutoGenConversableAgent",
-    "AutoGenConversableAgentThread",
-    "AzureAIAgent",
-    "AzureAIAgentSettings",
-    "AzureAIAgentThread",
-    "AzureAssistantAgent",
-    "BedrockAgent",
-    "BedrockAgentThread",
-    "ChatCompletionAgent",
-    "ChatHistoryAgentThread",
-    "OpenAIAssistantAgent",
-]
+_AGENTS = {
+    "Agent": ".agent",
+    "AgentResponseItem": ".agent",
+    "AgentThread": ".agent",
+    "AutoGenConversableAgent": ".autogen.autogen_conversable_agent",
+    "AutoGenConversableAgentThread": ".autogen.autogen_conversable_agent",
+    "AzureAIAgent": ".azure_ai.azure_ai_agent",
+    "AzureAIAgentSettings": ".azure_ai.azure_ai_agent_settings",
+    "AzureAIAgentThread": ".azure_ai.azure_ai_agent",
+    "BedrockAgent": ".bedrock.bedrock_agent",
+    "BedrockAgentThread": ".bedrock.bedrock_agent",
+    "ChatCompletionAgent": ".chat_completion.chat_completion_agent",
+    "ChatHistoryAgentThread": ".chat_completion.chat_completion_agent",
+    "AgentChat": ".group_chat.agent_chat",
+    "AgentGroupChat": ".group_chat.agent_group_chat",
+    "AzureAssistantAgent": ".open_ai.azure_assistant_agent",
+    "AssistantAgentThread": ".open_ai.open_ai_assistant_agent",
+    "OpenAIAssistantAgent": ".open_ai.open_ai_assistant_agent",
+}
+
+
+def __getattr__(name: str):
+    if name in _AGENTS:
+        submod_name = _AGENTS[name]
+        module = importlib.import_module(submod_name, package=__name__)
+        return getattr(module, name)
+    raise AttributeError(f"module {__name__} has no attribute {name}")
+
+
+def __dir__():
+    return list(_AGENTS.keys())

python/semantic_kernel/agents/__init__.pyi

@@ -0,0 +1,32 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+from .agent import Agent, AgentResponseItem, AgentThread
+from .autogen.autogen_conversable_agent import AutoGenConversableAgent, AutoGenConversableAgentThread
+from .azure_ai.azure_ai_agent import AzureAIAgent, AzureAIAgentThread
+from .azure_ai.azure_ai_agent_settings import AzureAIAgentSettings
+from .bedrock.bedrock_agent import BedrockAgent, BedrockAgentThread
+from .chat_completion.chat_completion_agent import ChatCompletionAgent, ChatHistoryAgentThread
+from .group_chat.agent_chat import AgentChat
+from .group_chat.agent_group_chat import AgentGroupChat
+from .open_ai.azure_assistant_agent import AzureAssistantAgent
+from .open_ai.open_ai_assistant_agent import AssistantAgentThread, OpenAIAssistantAgent
+
+__all__ = [
+    "Agent",
+    "AgentChat",
+    "AgentGroupChat",
+    "AgentResponseItem",
+    "AgentThread",
+    "AssistantAgentThread",
+    "AutoGenConversableAgent",
+    "AutoGenConversableAgentThread",
+    "AzureAIAgent",
+    "AzureAIAgentSettings",
+    "AzureAIAgentThread",
+    "AzureAssistantAgent",
+    "BedrockAgent",
+    "BedrockAgentThread",
+    "ChatCompletionAgent",
+    "ChatHistoryAgentThread",
+    "OpenAIAssistantAgent",
+]