fix: stabilize SFTP known_hosts generation in tests

[rmanibus]

Summary

• stop hand-building known_hosts lines in SFTP test helpers
• parse container host public keys with ssh.ParseAuthorizedKey
• generate canonical known_hosts entries with knownhosts.Line for e2e and package SFTP tests

Why

The ubuntu CI failure on main was caused by a malformed known_hosts entry in the SFTP test fixture, which broke the sftp-to-sftp integrity scenario during init.

Validation

• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./e2e -run 'TestCLI_Feature_IntegrityCheck'
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./pkg/store ./pkg/source -run 'TestSFTP(Store|Source)'
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache golangci-lint run ./e2e ./pkg/store ./pkg/source

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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