Add Rust caching to PR workflow to improve performance

[lukekim]

This pull request updates the .github/workflows/pr.yaml file to improve CI efficiency and reliability, mainly by introducing caching for Rust and Python dependencies and streamlining the Python build and test steps. The most important changes are grouped below.

Dependency Caching Improvements:

• Added a step to cache Rust dependencies using Swatinem/rust-cache@v2 in all relevant job sections to speed up Rust builds. [1] [2] [3]
• Enhanced Python dependency caching by configuring setup-uv to use a cache for dependencies and specifying a cache glob for python/pyproject.toml.

Build and Test Workflow Streamlining:

• Combined Python package build and test steps into a single job, running maturin develop in release mode and then executing tests with pytest, simplifying the workflow and ensuring tests run against the built package.