docs(cli): clarify AGENTS_DIR subdirectory behavior in adk web help

[vaibhav-patel]

Summary

The adk web / adk api_server CLI help describes AGENTS_DIR as a directory "where each subdirectory is a single agent", which implies every subdirectory is a valid agent. In practice this is not the case: AgentLoader.list_agents() (in src/google/adk/cli/utils/agent_loader.py) lists all non-hidden subdirectories (only excluding .-prefixed dirs and __pycache__) as candidate agents, with no validation that they contain agent files. Non-agent subdirectories (e.g. temp folders) therefore appear in the listing and are only skipped later, when loading them fails.

This was reported in #4425, where temp folders showed up in the adk web agent dropdown.

Changes

Reword the AGENTS_DIR help text in src/google/adk/cli/cli_tools_click.py (the adk web and adk api_server docstrings, plus the matching api_server argument comment) to describe the actual behavior:

• Each subdirectory is a candidate agent, expected to contain agent.py (or __init__.py) or root_agent.yaml.
• All non-hidden subdirectories are listed; subdirectories that are not valid agents are skipped when they fail to load.

Docs-only change; no behavior change. (The CLI reference docs at google.github.io/adk-docs are generated from these docstrings, so this also corrects the rendered reference.)

Verification

• python3 -m py_compile src/google/adk/cli/cli_tools_click.py passes.
• Confirmed the misleading "each subdirectory is a single agent" wording no longer appears in src/.

Fixes #4425.

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[vaibhav-patel]

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