Prevent Web UI from displaying non-agent directories

[gitsofaryan]

Resolves #5299

Description

This PR fixes an issue where the Web UI and underlying AgentLoader incorrectly listed non-agent directories (such as utils/, tmp/, etc.) in the agent discovery selector, resulting in a fractured user experience and ValueError loading errors.

Changes Made

1. AgentLoader Root Fix: Updated AgentLoader.list_agents() in src/google/adk/cli/utils/agent_loader.py. It now explicitly verifies that directories contain a recognized agent definition (root_agent.yaml, agent.py, or __init__.py) before appending them to the results list.
2. Endpoint Default: Changed the default value of the detailed query parameter in the /list-apps FastAPI endpoint (src/google/adk/cli/adk_web_server.py) from False to True. Because detailed=True implicitly leverages robust object instantiations, it functions as an immediate stop-gap backward-compatible fix for the Web UI without needing to force frontend compiled rebuilds.
3. Test Coverage:
   • Modified tests/unittests/cli/test_fast_api.py to ensure legacy assertions still evaluate ?detailed=false appropriately while validating the newly updated endpoint default.
   • Appended a new unit test test_list_agents_ignores_non_agent_directories to tests/unittests/cli/utils/test_agent_loader.py that identically emulates the Minimal Reproduction Code provided in the issue, ensuring we properly exclude irrelevant directories.

Testing Instructions

1. Run local unittests (pytest tests/unittests/cli/utils/test_agent_loader.py & pytest tests/unittests/cli/test_fast_api.py).
2. Locally spin up the Web UI using adk web . from a directory populated with both valid agent and non-agent subdirectories and verify that only the valid agents populate within the dropdown menu.

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[adk-bot]

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Thanks!

[rohityan]

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