[Khushbu677]

Implemented hex_to_ascii, a tool that converts hexadecimal strings into ASCII text. Added unit tests to verify valid input, invalid hexadecimal strings, and missing arguments.

• File is in tools/ directory
• File has header comments (# tool, # description, # author, # example)
• File has a run() function that returns a string
• Uses stdlib only (no external dependencies)
• Tested locally with python -m unittest tests.test_hex_to_ascii (7 tests passed)

Closes #624

[abduznik]

Reviewed. Matches issue #624 exactly: header conforms, run(*args)->str with args guard, ValueError/UnicodeDecodeError both return Error:, 7 tests with real assertions. Full suite green locally (557 passed). Merging.