fix(examples/keygen2): replace scanf with fgets to read into a fixed-size buffer

Fixes: #362 (closed)

This MR updates the input handling in keygen2.c to use fgets instead of scanf to safely read into a fixed-size buffer. It also introduces fflush to fix prompt visibility across all execution environments (terminals vs. pipes/redirections).

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code