test: add non-empty coverage for SyncResponse and FullScanResponse

[mubashirrao1122]

This PR adds additional test coverage for SyncResponse::is_empty and
FullScanResponse::is_empty.

It verifies responses are correctly reported as non-empty when:

• a chain update is present in SyncResponse
• last_active_indices is populated in FullScanResponse

These tests complement the existing default-empty coverage and help prevent
future regressions.

Tests:

• cargo test -p bdk_core --test test_spk_client

[Copilot]

Pull request overview

Adds non-empty test coverage for SyncResponse::is_empty and FullScanResponse::is_empty to prevent regressions where meaningful response data could be misclassified as empty.

Changes:

• Add a test asserting SyncResponse is non-empty when chain_update is set.
• Add a test asserting FullScanResponse is non-empty when last_active_indices is populated.

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[mubashirrao1122]

Hi! First contribution here added small tests for is_empty behavior. Happy to adjust anything if needed. Thanks