feat(byo): add lifespan hook for BYO agents

[onematchfox]

I'm looking for a way to be able to run background process(es) in a BYO agent. Whilst this is possible by setting up your own event loop, I thought it might be nicer(?) if we could hook into the existing FastAPI lifespan instead. So I figured I'd propose doing just that... That being said, I'm completely open to alternatives if anyone knows a better way of doing this 😄

[Copilot]

Pull Request Overview

This PR adds support for optional user-defined lifespan context managers in the KAgent ADK framework. Users can now define a lifespan function in their agent module that will be executed during the FastAPI application lifecycle, enabling custom setup and teardown logic.

Key Changes:

• Added a sample lifespan.py module demonstrating the lifespan pattern with setup and teardown logging
• Extended KAgentApp to accept an optional lifespan parameter and compose it with the built-in token service lifespan
• Modified the CLI to dynamically import and use user-defined lifespan functions from agent modules

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
python/samples/adk/basic/basic/lifespan.py	Sample implementation of user-defined lifespan with logging
python/samples/adk/basic/basic/init.py	Exports the new lifespan function for discoverability
python/packages/kagent-adk/src/kagent/adk/cli.py	Dynamically imports lifespan from agent modules and passes it to KAgentApp
python/packages/kagent-adk/src/kagent/adk/_a2a.py	Adds lifespan parameter and composition logic to integrate user-defined and built-in lifespans

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[Copilot]

The dynamic import uses the agent name directly without adjusting sys.path to include working_dir. If the agent module is in a subdirectory (e.g., working_dir/name), this import may fail. Consider using sys.path.insert(0, working_dir) before the import to ensure the module can be found, similar to how AgentLoader handles the working directory.

Suggested change

	try:
	try:
	sys.path.insert(0, working_dir)

[Copilot]

The error message 'Failed to load agent module for lifespan' is too generic and doesn't indicate which module failed to load. Consider including the module name in the error message: f\"Failed to load agent module '{name}' for lifespan\" to aid in debugging.

Suggested change

	logger.exception("Failed to load agent module for lifespan")
	logger.exception(f"Failed to load agent module '{name}' for lifespan")

[Copilot]

The build_local() method doesn't compose the user-defined lifespan with any base lifespan like build() does with token_service.lifespan(). While this may be intentional for local mode, it creates inconsistency. If users define a lifespan expecting composition behavior (e.g., wrapping around a base lifespan), it won't work in local mode. Consider either using _compose_lifespan() with a no-op base lifespan or documenting this behavioral difference.

Suggested change

	if self._lifespan is not None:
	app = FastAPI(lifespan=self._lifespan)
	else:
	app = FastAPI()
	from contextlib import asynccontextmanager
	@asynccontextmanager
	async def noop_lifespan(app):
	yield
	composed_lifespan = self._compose_lifespan(noop_lifespan)
	app = FastAPI(lifespan=composed_lifespan)

[Copilot]

Import of 'sys' is not used.

Suggested change

import sys