chore: bump codecov-action and checkout to v7

[rustatian]

Brings codecov-action and actions/checkout from v6 to v7, matching the rest of the fleet. No behaviour change — this repo's codecov job is already correct and reports 78%.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.38%. Comparing base (8e2352d) to head (2a7e3f4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   78.38%   78.38%           
=======================================
  Files           7        7           
  Lines         495      495           
=======================================
  Hits          388      388           
  Misses         80       80           
  Partials       27       27           

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