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