test: cover web reload agent observer setup

[he-yufeng]

Summary

• remove redundant function-local ApiServer imports in get_fast_api_app
• add a regression test for web=True plus reload_agents=True while DevServer is available

Fixes #5765

To verify

• .\.venv\Scripts\python.exe -m py_compile src\google\adk\cli\fast_api.py tests\unittests\cli\test_fast_api.py
• .\.venv\Scripts\python.exe -m pytest tests/unittests/cli/test_fast_api.py::test_web_reload_agents_uses_module_api_server_import -q -p no:cacheprovider
• uv run --extra dev pyink --check src\google\adk\cli\fast_api.py tests\unittests\cli\test_fast_api.py
• git diff --check

Notes

• uv run --extra test ... currently cannot resolve the locked test environment on Windows because yarl==1.24.1 has no win_amd64 wheel or source distribution.

[adk-bot]

Response from ADK Triaging Agent

Hello @he-yufeng, thank you for creating this PR!

This PR looks great and addresses issue #5765! To fully follow our contribution guidelines, could you please:

• Provide the logs or a screenshot of the passed test/verification output (e.g., the pytest output for the regression test you added).

This will help reviewers understand and approve your changes much faster. Thank you!

[rohityan]

Hi @he-yufeng , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[he-yufeng]

Rebased onto current main, added the required future annotations import for the changed non-test Python file, and re-ran the focused checks.

Local validation:

• python -m pytest tests/unittests/cli/test_fast_api.py::test_web_reload_agents_uses_module_api_server_import -q --basetemp .tmp/pytest-5769-focused -> 1 passed
• python -m pyink src/google/adk/cli/fast_api.py tests/unittests/cli/test_fast_api.py --check -> passed
• python -m py_compile src/google/adk/cli/fast_api.py tests/unittests/cli/test_fast_api.py -> passed
• git diff --check -> passed

I also checked the changed non-test Python file for the required from __future__ import annotations import. A full local file-level pytest run hits Windows tempfile cleanup locks in unrelated tests, so I kept the local validation focused on this PR's regression case.

[he-yufeng]

Rebased this onto current main. The implementation conflict is now resolved by upstream main (the web server class selection already uses the module-level DevServer/ApiServer imports), so this PR now only keeps the focused regression coverage for the reload observer path.

Local validation after the rebase:

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pytest tests\unittests\cli\test_fast_api.py::test_web_reload_agents_uses_module_api_server_import -q --basetemp .tmp\pytest-5769-rebase
# 1 passed, 5 warnings

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pyink tests\unittests\cli\test_fast_api.py --check
PYTHONUTF8=1 .\.venv\Scripts\python.exe -m py_compile tests\unittests\cli\test_fast_api.py
git diff --check upstream/main..HEAD