chore(rust): add just target for cargo-deny

[ajsutton]

Summary

• Add a deny just target to rust/justfile that runs cargo deny --all-features check all
• Update CI (rust-ci.yml) to use just deny instead of invoking cargo deny directly
• Update docs/ai/rust-dev.md to reference the new target

Stacked on #19518.

Test plan

• Run cd rust && just deny locally — should produce same output as cargo deny --all-features check all
• Verify CI rust-deny job passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.5%. Comparing base (1e2b976) to head (fe6b044).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #19520     +/-   ##
==========================================
- Coverage     75.5%    75.5%   -0.1%     
==========================================
  Files          675      675             
  Lines        71571    71571             
==========================================
- Hits         54079    54070      -9     
- Misses       17348    17357      +9     
  Partials       144      144             

Flag	Coverage Δ
cannon-go-tests-64	66.4% <ø> (ø)
contracts-bedrock-tests	80.2% <ø> (ø)
unit	75.5% <ø> (-0.1%)	⬇️

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see 5 files with indirect coverage changes

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