Upgrade Rust nightly toolchain to 2025-12-15

## Summary
Upgrade the Rust nightly toolchain from `nightly-2025-05-09` to `nightly-2025-12-15` to incorporate the latest compiler improvements and bug fixes.

## Motivation
- Keep Rust toolchain up-to-date with recent improvements
- Benefit from compiler optimizations and bug fixes in the past 220 days
- Maintain compatibility with the latest Rust ecosystem dependencies

## Changes
- Update `rust-toolchain.toml`: `nightly-2025-05-09` → `nightly-2025-12-15`
- No breaking changes to existing code expected

Open rust-toolchain.toml and locate the nightly-2025-05-09 toolchain entry. Update it to nightly-2025-12-15, then verify that the project uses the requested nightly toolchain and that existing code remains compatible. Done means the file contains the new pin without unrelated changes.

Written by the indexing model from the issue text.