Setup dependabot to do rust-toolchain updates

[Jefffrey]

apparently dependabot can do this for us

• https://github.blog/changelog/2025-08-19-dependabot-now-supports-rust-toolchain-updates/

though not sure how useful it may be since theres usually clippy fixes that accompany this 🤔

worth enabling this so at least there wont be cases of us forgetting to bump it when a new version drops?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.70%. Comparing base (e8a65f2) to head (233fde3).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23633      +/-   ##
==========================================
+ Coverage   80.66%   80.70%   +0.03%     
==========================================
  Files        1095     1095              
  Lines      372294   372542     +248     
  Branches   372294   372542     +248     
==========================================
+ Hits       300324   300642     +318     
+ Misses      54055    53934     -121     
- Partials    17915    17966      +51     

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[Jefffrey]

driveby since we might as well label our github action/pip bumps with this label too