Test cache legacy migration coverage

[ktsaou]

Summary

• Add behavior coverage for bash-era cache migration through LoadLegacy and LoadWithMigration.
• Cover shell quoting, legacy field migration, malformed declarations, JSON-cache precedence, missing legacy fallback, and invalid legacy errors.
• Keep production code unchanged.

Quality impact

• pkg/cache coverage: 78.3% -> 88.8%.
• Root coverage: 73.4% -> 73.8%.
• Production large functions remain at 0.

Validation

• go test ./pkg/cache
• go test -count=1 -coverprofile=/tmp/update-ipsets-cache-slice29.cover -covermode=atomic ./pkg/cache
• go run ./tools/archposture -root . > /tmp/update-ipsets-archposture-slice29.json
• make lint
• make staticcheck
• make golangci-lint
• CI=true make coverage
• make test-strict
• git diff --check

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 50 complexity · 9 duplication

Metric	Results
Complexity	50
Duplication	9

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AI Reviewer: first review requested successfully. AI can make mistakes. Always validate suggestions.

_{TIP This summary will be updated as you push new changes.}