fix(tashfeer): prevent infinite loop and event loop hang on invalid levels (fix #35)

[AbdelrahmanBayoumi]

Closes #35

Summary of Changes:

• Added validateTashfeerLevel to validate input encryption levels across tashfeer, tashfeerBannedWords, and toOldArabicAndTashfeerBannedWords.
• Guards getRandomIndexes with bounded index sampling (randomIndexes.size < count) and handles boundary cases (wordLength <= 0).
• Prevents infinite loops / Event Loop lockups caused by negative, fractional, or non-finite numbers.
• Added comprehensive unit tests in tests/edge-cases.test.ts covering edge cases, invalid inputs, and performance boundaries.
• All 80 tests passing, linting clean, and TypeScript checks pass.