Refactor ASN database processing

[ktsaou]

Summary

• move ASN provider lifecycle handling into focused helpers
• keep processASNDatabases responsible for orchestration, cancellation, and close-on-error cleanup
• cover invalid format, missing database, staged archive, successful load, and open-failure paths

Validation

• go test ./pkg/engine
• go test -coverprofile=/tmp/update-ipsets-engine-slice25.cover -covermode=atomic ./pkg/engine
• go run ./tools/archposture -root . > /tmp/update-ipsets-archposture-slice25.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 70 complexity · 5 duplication

Metric	Results
Complexity	70
Duplication	5

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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.}