Prevent panic by Number::modulo

[jaylorch]

In Number::modulo, it calls Number::ints_to_bigint, which could panic. The reason is that calling .to_integer() isn't enough to guarantee that .to_bigint_owned() will return Some, but Number::ints_to_bigint assumes it will and calls unwrap(). In particular, it might be that it's a float corresponding to an integer that's larger than F64_SAFE_INTEGER. The fix is to not call Number::ints_to_bigint (and indeed to delete that entire function, which is only used in this one place), and instead only call unwrap when Some is returned.

A concrete example of interpreter input that would cause a panic is 100000000000000000e-1 % 2. This PR prevents that panic.

[Copilot]

Pull request overview

This PR hardens Number::modulo to avoid a panic when operands are floats that are mathematically integral but not exactly representable as integers (e.g., magnitude beyond 2^53), ensuring modulo returns an error instead of panicking.

Changes:

• Removes the ints_to_bigint helper that unconditionally unwrap()ed bigint conversions and could panic.
• Updates Number::modulo to only proceed when both operands can be safely converted to BigInt, otherwise returning modulo on floating-point number.
• Adds regression coverage via an interpreter YAML case and a Rust unit test for large integral floats.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/number.rs	Reworks Number::modulo to avoid panic on failed float→bigint conversion; removes the panic-prone helper; adds unit tests.
tests/interpreter/cases/builtins/numbers/mod.yaml	Adds a regression test for modulo on an integral-but-too-large float that previously could panic.

[anakrish]

Thanks for identifying and fixing this @jaylorch. Once the verus formalization of number is done, I'll want to revisit number.

I don't think the official OPA semantics of numbers is that well defined (e.g large integers) and might not be worth being compatible with if we can devise a simpler semantics that policies could be written to.

[jaylorch]

I had to rebase, which apparently requires another approval.