fix(agent_loader): support flat-module agents in _determine_agent_language

[RudrenduPaul]

Problem

_determine_agent_language() in agent_loader.py handles only 3 of the 4 agent structures that load_agent() supports. When a user structures their agent as a flat module (agents_dir/my_agent.py with no subdirectory), load_agent() successfully loads it, but _determine_agent_language() raises ValueError — causing list_agents_detailed() (used by adk web and adk api_server) to fail with a crash for that agent.

The four load patterns load_agent() supports:

• {name}/agent.py ✅ detected
• {name}/__init__.py ✅ detected
• {name}/root_agent.yaml ✅ detected
• {name}.py flat module ❌ raised ValueError — this PR fixes this

Changes

• src/google/adk/cli/utils/agent_loader.py: Add elif (base_path.parent / f"{agent_name}.py").exists(): return "python" branch to _determine_agent_language()
• src/google/adk/cli/cli_tools_click.py: Update docstrings for cli_web and cli_api_server that incorrectly stated agents must be subdirectories with __init__.py and agent.py
• tests/unittests/cli/utils/test_agent_loader.py: Add TestDetermineAgentLanguage class with 5 tests covering all 4 supported patterns plus the unrecognized-structure error case

Testing plan

New TestDetermineAgentLanguage class added with tests for:

1. Flat-module agent (agents_dir/my_agent.py) → returns "python" ✅
2. Subdirectory with agent.py → returns "python" ✅
3. Subdirectory with __init__.py → returns "python" ✅
4. Subdirectory with root_agent.yaml → returns "yaml" ✅
5. Unrecognized structure → raises ValueError ✅

Full CI run required to verify (local run requires google.genai dependency installation).

[RudrenduPaul]

I have read the CLA Document and I hereby sign the CLA

[rohityan]

Hi @RudrenduPaul ,Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @DeanChensj , can you please review this.