Add combined contract attribute

[robjtede]

Summary

• add a new #[contract(...)] attribute for grouping contract clauses in one macro invocation
• reuse the existing contract parser/codegen path for requires, ensures, invariant, and mode-prefixed clauses
• update the issue clippy reports unused imports when using both requires and ensures #9 regression to import only contract under deny(unused_imports)

Closes #9.

Validation

• cargo +1.65.0 test --test ui
• just clippy
• just test

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🧪 Generate unit tests (beta)

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• Commit unit tests in branch codex-add-combined-contract-attribute

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