feat(cli): add --lifespan option to api_server command

[kemurayama]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: #issue_number
• Related: adk command option to enable or disable the correlation of structured logs with Cloud Trace. #1683

(Note: If there is no specific issue, please leave the above blank or remove them, and fill in section 2 below.)

2. Or, if no issue exists, describe the change:

Problem:
Currently, there is no way to register a lifespan handler (startup and shutdown logic) directly from the adk api_server CLI command in v1 and v2. While the underlying get_fast_api_app function accepts a lifespan argument, the CLI command does not expose it, preventing users from running custom setup or teardown logic for their agents.

Solution:
This PR introduces a --lifespan option to the adk api_server CLI command. This allows users to specify a lifespan context manager dynamically via its import path (e.g., path.to.module.lifespan_handler).

• Added --lifespan option to the api_server command definition.
• Implemented _load_lifespan_handler helper to dynamically import the lifespan handler using its string module path.
• Updated cli_api_server to dynamically load the lifespan handler and pass it down to get_fast_api_app.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed pytest results:
Added test_cli_api_server_passes_lifespan in tests/unittests/cli/utils/test_cli_tools_click.py to verify that the CLI correctly parses --lifespan, loads the module, and passes the handler to get_fast_api_app.

Command:

pytest tests/unittests/cli/utils/test_cli_tools_click.py -k test_cli_api_server_passes_lifespan