Started adding tests for uncovered code paths that looked worth covering.

/claim #560

Rationale for this change

Improve coverage for uncovered production paths with focused tests rather than broad or redundant assertions.

What changes are included in this PR?

• added coverage for planner expression conversion paths
• added coverage for slice execution helper paths
• expanded multilinear extension coverage across more column variants
• added timestamp unit conversion and display coverage
• expanded coverage in database, proof, and arrow paths that were still uncovered
• added utility coverage for generate-parameters
• added utility coverage for commitment-utility

Are these changes tested?

Yes.