Changed modulusLength from 2048 to 4096 in generateKeypair() method
following NIST recommendations for long-term security. Added test to
verify generated keys meet the minimum size requirement.
Security improvement: RSA-4096 provides better protection against
future cryptographic attacks as computational power increases.
Fixes#288
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>