fix: Add overflow protection to Fibonacci implementation

[DeVikingMark]

Description:

• Fix edge case handling for n=1
• Add overflow protection using checked_add
• Return u32::MAX on overflow instead of panicking

It prevents potential integer overflow vulnerabilities in the zkVM test suite while maintaining backwards compatibility.

[sjudson]

@DeVikingMark Thanks for the fixes. As part of our new licensing, we require signing a contributor license agreement in order to contribute to the the codebase.

If you rebase this PR off main the bot will activate and walk you through the (very straightforward) process to sign the CLA.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[DeVikingMark]

I have read the CLA Document and I hereby sign the CLA

[DeVikingMark]

@DeVikingMark Thanks for the fixes. As part of our new licensing, we require signing a contributor license agreement in order to contribute to the the codebase.

If you rebase this PR off main the bot will activate and walk you through the (very straightforward) process to sign the CLA.

@sjudson yeah sure, I signed it

[sjudson]

Retracting my review, seems like the tests that use this program are now failing.

[DeVikingMark]

Retracting my review, seems like the tests that use this program are now failing.

@sjudson May I ask you to have a look at it now? I suppose it should be fine, but if it's not, I'll try to seek deeper the problem

[sjudson]

Thanks for the fix.