fix: validate CrowdSec archive entry names with a standard local-path check

[Wikid82]

Summary

A local CodeQL Go scan reported a new go/zipslip result in extractTarGz (backend/internal/crowdsec/hub_sync.go) after #1535. That PR replaced a HasPrefix(name, "..") guard with a custom helper (to accept names such as ..foo), and CodeQL cannot see through a custom helper.

Entry names are now checked with the standard filepath.IsLocal before they reach the path join. Behaviour is unchanged for real archives: ..foo, a/..b, ./a.txt are accepted; ../x, a/../../x and absolute paths are rejected. One edge case differs: an empty entry name is now rejected instead of cleaning to .. The containment check, symlink-safe file creation and engine-owned-path skip are unchanged.

Testing

• go test ./internal/crowdsec/..., go vet, full golangci-lint config: clean.
• New test covers ./ directory entries, ./a.txt and the empty-name case.
• Local CodeQL: Go "4 suppressed, 0 blocking, 4 total" (the existing suppressions), JS 0 results; no new suppression entry was needed.

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[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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