fix(foundation): should error on attempt to convert negative bigint to buffer

[dbanks12]

Description

Should error on attempt to convert negative bigint to buffer. Encountered this in a branch when intentionally using leafIndex -1 to flag commitments as transient.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• The branch has been merged or rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.