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.

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[FeodorFitsner]

The error handler currently suppresses failures from both os.chmod() and the deletion retry, even when ignore_errors=False. For example, a Windows file that is still locked after clearing its read-only attribute will remain in the output directory, but cleanup returns successfully. The build can then copy new artifacts into that directory and report success with stale files still present.

Could we let retry failures propagate by removing the blanket try/except?

def _remove_readonly(func, file_path, _):
    os.chmod(file_path, stat.S_IWRITE)
    func(file_path)

shutil.rmtree() already handles ignore_errors=True without invoking this callback, so this preserves that option while making the default behavior fail visibly when cleanup cannot complete.

A regression test should simulate deletion failing both initially and after chmod, and assert that rmtree() raises PermissionError. I reproduced the silent-success behavior with a simulated locked file through both callback branches; native Windows behavior was not tested.