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.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23633   +/-   ##
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  Coverage        ?   80.65%           
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  Files           ?     1086           
  Lines           ?   366097           
  Branches        ?   366097           
=======================================
  Hits            ?   295273           
  Misses          ?    53230           
  Partials        ?    17594           

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