chore: use Rust 1.97 cargo warnings

[nyurik]

• I agree to follow the project's code of conduct.
  - [ ] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.81%. Comparing base (7702972) to head (d70169b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #47   +/-   ##
=======================================
  Coverage   73.81%   73.81%           
=======================================
  Files           4        4           
  Lines         317      317           
=======================================
  Hits          234      234           
  Misses         83       83           

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[Copilot]

Pull request overview

This PR updates the project’s CI “warnings as errors” configuration in the justfile to rely on Cargo’s warnings control instead of injecting -D warnings via RUSTFLAGS/RUSTDOCFLAGS.

Changes:

• Replace RUSTFLAGS/RUSTDOCFLAGS CI exports with CARGO_BUILD_WARNINGS (defaulting to deny in CI).
• Update env-info output to report CARGO_BUILD_WARNINGS instead of the removed flags.

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