fix(malachite-app): harden response capacity check with safe arithmetic in GetDecidedValues

[forumevi]

Summary

This PR replaces manual arithmetic logic and #[allow(clippy::arithmetic_side_effects)] suppression in GetDecidedValues handler with idiomatic, bounds-checked arithmetic (checked_sub + map_or).

Context & Motivation

In crates/malachite-app/src/handlers/get_decided_values.rs, the response size evaluation relied on raw subtraction guarded by conditional short-circuiting:

#[allow(clippy::arithmetic_side_effects)]
if raw_bytes_len > max_response_size || total_bytes.as_u64() > max_response_size.as_u64() - raw_bytes_len.as_u64()

While designed to prevent underflow, relying on manual suppression of Clippy arithmetic warnings introduces unnecessary maintenance risks and potential edge-case arithmetic panics during boundary value conversions.

Solution

• Refactored the capacity subtraction to use as_u64().checked_sub(...).
• Leveraged map_or(true, ...) to gracefully handle non-sufficient remaining capacity without triggering unchecked subtraction.
• Removed the #[allow(clippy::arithmetic_side_effects)] attribute, aligning the handler with strict production resilience standards.

Testing

• Verified through crate test suite (cargo test -p malachite-app --lib) — 400/400 tests passing.
• Verified compilation and Clippy adherence with zero warnings.