fix(cli): use Optional[List[str]] in cleanup_unused_files to fix parser error

[syzayd]

Problem

When the ADK web server runs and the LLM calls the cleanup_unused_files built-in tool, the automatic function calling system fails to parse its optional parameter type annotations:

Error in event_generator: Failed to parse the parameter
file_patterns: List[str] | None = None of function
cleanup_unused_files for automatic function calling

The root cause is that file_patterns and exclude_patterns use the PEP 604 union syntax (list[str] | None) which ADK's function schema builder cannot handle.

Fixes #3591

Fix

All sibling tools in src/google/adk/cli/built_in_agents/tools/ already use Optional[List[str]] from typing (see write_files.py, delete_files.py, read_files.py). This PR aligns cleanup_unused_files.py with that established pattern:

• Add from typing import List and from typing import Optional
• Replace list[str] | None with Optional[List[str]] on file_patterns and exclude_patterns
• Change used_files: list[str] to used_files: List[str] for style consistency

Testing

The fix can be verified by running adk web, opening the agent editor, and confirming the cleanup_unused_files tool is invoked without the parser warning appearing in the console.

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