Restore strict Clippy cleanliness

[juangaitanv]

Summary

Restore strict Clippy cleanliness on the current Rust toolchain without changing runtime behavior.

• move the SARIF test module after production items
• use std::slice::from_ref in the suppression test
• replace a length comparison with is_empty
• remove a redundant terminal return

The current toolchain surfaced all four diagnostics under -D warnings, blocking the strict lint gate.

Related issue

None.

Checklist

• make ci passes locally (the same command CI runs — see CONTRIBUTING.md)
• Ran make bootstrap once so pre-commit/pre-push hooks are active
• Added/updated tests for the change
• Updated docs/rules where relevant

Verification

make ci exited 0:

• strict Clippy passed
• format check passed
• dependency audit passed
• 200 tests passed
• Cucumber acceptance passed
• coverage completed
• architecture check passed

Complexity and CRAP reported their existing advisory backlog and did not block the gate.

[juangaitanv]

Superseded by #33, which includes these Clippy fixes in the Rust 2024 migration.