fix(build): handle read-only files on Windows in rmtree to prevent PermissionError (fixes #6808)

[eminsk]

Summary of Changes

This PR fixes PermissionError: [WinError 5] Access denied on Windows during flet build windows when cleaning up output directories that contain read-only files (such as .git pack/index objects).

Fixes #6808

Root Cause

In flet_cli/commands/build_base.py:

if self.out_dir.exists():
    shutil.rmtree(str(self.out_dir))

On Windows, standard shutil.rmtree without an error handler fails with PermissionError when encountering files with the read-only attribute (stat.S_IREAD), which frequently occur inside .git/objects/pack/ or other build artifacts.

Solution

1. Added rmtree to flet.utils.files (and re-exported in flet.utils) with an error handler that removes the read-only attribute (os.chmod(path, stat.S_IWRITE)) before retrying deletion. Supports both Python 3.12+ (onexc) and prior versions (onerror).
2. Updated build_base.py to use rmtree instead of direct shutil.rmtree for directory cleanup.
3. Added unit tests in sdk/python/packages/flet/tests/test_files.py.

Summary by Sourcery

Make Windows build directory cleanup resilient to read-only files.

Bug Fixes:

• Prevent Windows build cleanup from failing when output directories contain read-only files.
• Handle removal of nonexistent directories without raising errors.

Enhancements:

• Provide a shared recursive directory removal utility with read-only file handling across supported Python versions.

Tests:

• Add coverage for deleting read-only files and nonexistent directories.

[CLAassistant]

All committers have signed the CLA.

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