test: skip unavailable optional 7z format

[nixfred]

What was wrong

The archive suite always exercised 7z even when the optional 7zip package was absent and Flea correctly omitted 7z from its runtime format list. That made a supported dependency configuration report a false test failure.

Fix

Derive the optional 7z round trip from the backend's live formats response while keeping required tar.zst and zip coverage unconditional.

Verification

• reproduced on a system without 7zip
• FLEA_FIXTURE_ROOT=/tmp/flea-sandbox ./tests/archive.sh: all checks passed
• bash -n tests/archive.sh
• file-budget gate and git diff --check pass

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 979ad25 and 3d56a53.

📒 Files selected for processing (1)

• tests/archive.sh

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