fix: replace std::exit with std::invalid_argument in morse_code

[https-hari]

Fixes #3176

Problem

char_to_morse and morse_to_char both called std::exit(0) on invalid
input. This kills the entire process with exit code 0 (success), making
it impossible for callers to handle bad input gracefully via try/catch.

Fix

Replaced both std::exit(0) calls with throw std::invalid_argument(...).
Added #include <stdexcept> accordingly.

Tests

Added two test cases verifying that:

• char_to_morse throws std::invalid_argument on an unsupported character
• morse_to_char throws std::invalid_argument on an unrecognised morse string