[test] Support manually trigger and wait for KV snapshots for FlussClusterExtension

[wuchong]

Purpose

Linked issue: close #xxx

This pull request refactors snapshot-related test utilities across multiple integration test cases to improve consistency and reliability. The main change is replacing manual snapshot waiting logic and configuration with standardized methods provided by FLUSS_CLUSTER_EXTENSION, and removing unnecessary configuration of snapshot intervals for testing purposes.

Snapshot Triggering and Waiting Refactor:

• Replaced calls to custom snapshot waiting methods (e.g., waitUntilAllSnapshotFinished, waitUntilSnapshotFinished, and waitUntilAllBucketFinishSnapshot) with standardized calls to FLUSS_CLUSTER_EXTENSION.triggerAndWaitSnapshot and FLUSS_CLUSTER_EXTENSION.triggerAndWaitSnapshots in test cases such as FlinkTableSourceITCase, KvSnapshotBatchScannerITCase, and FlussAdminITCase. This improves test reliability and code maintainability.

Configuration Cleanup:

• Removed test-specific configuration for snapshot intervals (KV_SNAPSHOT_INTERVAL) from cluster configuration initialization in several test classes, relying instead on default settings and explicit snapshot triggering. This reduces test configuration complexity and potential flakiness.

Code Simplification:

• Removed unused imports and redundant utility methods related to snapshot waiting, such as waitUntilAllSnapshotFinished and waitUntilAllBucketFinishSnapshot, further streamlining test code.

Overall, these changes standardize snapshot handling in tests, reduce configuration overhead, and simplify the codebase, making the tests easier to maintain and less error-prone.

Brief change log

Tests

API and Format

Documentation

[luoyuxia]

@wuchong Thanks for the clean up. LGTM!