fix: handle rust toolchain versions in path selection

[loonghao]

Summary

• Treat Rust compiler/toolchain versions as rustup-managed when selecting vx store paths
• Avoid warning that a Rust toolchain version is missing from the rustup-versioned store
• Add regression coverage for Rust toolchain version detection

Tests

• vx cargo fmt --all -- --check
• vx cargo test -p vx-resolver --test project_config_tests
• vx cargo test -p vx-resolver
• vx cargo clippy -p vx-resolver --all-targets -- -D warnings

[codecov]

Codecov Report

❌ Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.23%. Comparing base (363d986) to head (03ae3fa).
⚠️ Report is 86 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/vx-resolver/src/executor/environment.rs	40.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
- Coverage   54.26%   54.23%   -0.04%     
==========================================
  Files         377      382       +5     
  Lines       66876    67699     +823     
==========================================
+ Hits        36289    36715     +426     
- Misses      30587    30984     +397     

Flag	Coverage Δ
unittests	54.23% <79.31%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/vx-resolver/src/executor/project_config.rs	87.50% <100.00%> (+0.23%)	⬆️
crates/vx-resolver/src/executor/environment.rs	52.21% <40.00%> (-0.23%)	⬇️

... and 70 files with indirect coverage changes

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