fix(framework): Improve zip extraction robustness in flwr install

[danieljanes]

This PR fixes a potential ZIP path traversal risk in FAB install:

• zipf.extractall(tmpdir) was used directly in install.py (line 122)
• In contrast, flwr new already used safe extraction with path checks in new.py (line 145)

This PR refactors both to use a common function to safely extract zip files.

[Copilot]

Pull request overview

This PR improves the robustness of ZIP extraction in the Flower CLI by consolidating security checks into a reusable utility function to prevent zip-slip path traversal attacks. The refactoring extracts the previously private _safe_extract_zip function from new.py into a shared archive_utils.py module, making it available for both the flwr new and flwr install commands.

Changes:

• Created new archive_utils.py module with safe_extract_zip function for secure ZIP extraction
• Refactored install.py to use the centralized secure extraction function instead of unsafe zipfile.extractall()
• Refactored new.py to use the shared utility instead of a local implementation
• Added comprehensive test coverage for zip-slip protection in both install and new commands

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
framework/py/flwr/cli/archive_utils.py	New module containing the centralized safe_extract_zip function to prevent zip-slip attacks
framework/py/flwr/cli/install.py	Updated to use safe_extract_zip instead of unsafe zipfile.extractall()
framework/py/flwr/cli/new/new.py	Removed duplicate _safe_extract_zip implementation and imports the shared utility
framework/py/flwr/cli/install_test.py	New comprehensive tests for safe ZIP extraction with path traversal protection
framework/py/flwr/cli/new/new_test.py	Added test for zip-slip protection in the app download flow

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