Test coverage: network errors, timeouts, and concurrent pagination for new operations

[leogdion]

Summary

PR #204 added lookupZones, fetchRecordChanges, and uploadAssets with happy-path and validation tests, but several error and edge-case scenarios are not yet covered.

Missing tests

• Network errors and timeouts for all three new operations
• fetchZoneChanges with an invalid or expired sync token
• Concurrent calls to fetchAllRecordChanges() (data race check)
• discoverUserIdentities with an invalid email format in the lookup info

Notes

Identified in PR #204 review. Deferred to a follow-up PR to keep coverage additions focused.