Add secure_file_reader utility and tests

[Mwessc]

This commit introduces a new Python utility function, secure_file_reader, located in src/secure_file_utils.py. This function demonstrates secure coding principles for reading files, including:

• Robust input sanitization (stripping whitespace, checking for empty filenames).
• Prohibition of dangerous characters and sequences (e.g., '../', absolute paths, null bytes).
• Enforcement of allowed file extensions.
• Path whitelisting by ensuring the resolved absolute path of a requested file is within a defined secure base directory.

The function is designed for simulation and educational purposes, showing how to build file handling logic with security in mind. It includes detailed comments explaining each security principle.

Comprehensive unit tests have been added in tests/test_secure_file_utils.py using the unittest framework. These tests cover various scenarios, including valid access, attempted exploits (path traversal, null byte injection, etc.), and handling of disallowed extensions.

A .gitignore file has also been added to exclude common Python cache files from the repository.

Description

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Checklist

• I've added the prompt in the correct section.
• The prompt is helpful, concise, and clear.
• I've double-checked the markdown formatting.