fix: allow builder assistant special app name

[he-yufeng]

Summary

• fix adk web runner construction for internal special agents such as __adk_agent_builder_assistant
• keep normal user app-name validation unchanged
• add a regression test for the builder assistant sentinel name

Fixes #5877.

To verify

• C:\dev\GITHUB-clean\adk-python\.venv\Scripts\python.exe -m pytest tests\unittests\cli\test_fast_api.py -q -k "special_agent_name or bigquery" — 2 passed, 73 deselected, 8 warnings
• C:\dev\GITHUB-clean\adk-python\.venv\Scripts\python.exe -m pytest tests\unittests\apps\test_apps.py -q -k "app_name_must_be_valid or path_like" — 1 passed, 15 deselected, 4 warnings
• C:\dev\GITHUB-clean\adk-python\.venv\Scripts\python.exe -m pyink --check src\google\adk\cli\api_server.py tests\unittests\cli\test_fast_api.py — passed, 2 files left unchanged
• git diff --check -- src\google\adk\cli\api_server.py tests\unittests\cli\test_fast_api.py — passed

[adk-bot]

Response from ADK Triaging Agent

Hello @he-yufeng, thank you for submitting this pull request!

This PR is a bug fix and addresses issue #5877. While you have provided a detailed "To verify" section with the commands, could you please provide:

1. Logs or Screenshots: Please provide logs or screenshots of the verification run or adk web behavior after the fix is applied.
2. Pytest Results: Please include a summary of passed pytest results as outlined in our unit testing requirements.

Providing this verification evidence helps our maintainers review and merge your PR more efficiently. For more details, you can refer to the Contribution Guidelines.

Thank you!

[he-yufeng]

Rebased onto current upstream/main and refreshed the PR verification evidence. The targeted pytest commands now report 2 passed and 1 passed respectively; pyink and diff-check also pass.