Add unit tests for Rust resolver

[Jaydeep869]

Description

The Rust resolver currently lacks unit tests. With PRs #2 and #3 adding tests for JavaScript, Python, and Go resolvers, we should complete test coverage for the Rust resolver as well.

Motivation

• Ensures Rust package resolution works correctly
• Prevents regressions in Cargo path parsing
• Maintains consistency with other ecosystem resolvers
• Critical for production readiness

Scope

• Create pkg/resolver/rust_test.go
• Test Cargo registry path parsing (e.g., registry/src/github.com-1ecc6299db9ec823/serde-1.0.130)
• Test version extraction from paths
• Test PURL generation (pkg:cargo/serde@1.0.130)
• Test edge cases (invalid paths, malformed versions)
• Test package file filtering
• Minimum 5-8 comprehensive test cases

Example Test Cases

1. Valid Cargo registry path → correct package info
2. Scoped package name handling
3. Version extraction from path
4. PURL format validation
5. Invalid path handling
6. Package file filter matching