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

ADK's automatic function calling system cannot parse the PEP 604 union
syntax `list[str] | None` when building the tool schema for Gemini.
This causes the following error every time the ADK web server runs:

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

All sibling tools in this directory (delete_files.py, read_files.py,
write_files.py, etc.) already use `Optional[List[str]]` from `typing`.
This commit aligns cleanup_unused_files.py with that established pattern.

Changes:
- Add `from typing import List` and `from typing import Optional`
- Replace `list[str] | None` with `Optional[List[str]]` on both optional
  parameters (file_patterns, exclude_patterns)
- Change `used_files: list[str]` to `used_files: List[str]` for style
  consistency with sibling tools

Fixes #3591

Author: syzayd

src/google/adk/cli/built_in_agents/tools/cleanup_unused_files.py

@@ -17,6 +17,8 @@
 from __future__ import annotations
 
 from typing import Any
+from typing import List
+from typing import Optional
 
 from google.adk.tools.tool_context import ToolContext
 
@@ -25,10 +27,10 @@
 
 
 async def cleanup_unused_files(
-    used_files: list[str],
+    used_files: List[str],
     tool_context: ToolContext,
-    file_patterns: list[str] | None = None,
-    exclude_patterns: list[str] | None = None,
+    file_patterns: Optional[List[str]] = None,
+    exclude_patterns: Optional[List[str]] = None,
 ) -> dict[str, Any]:
   """Identify and optionally delete unused files in project directories.