[fix](fe) Deduplicate backends in ADMIN CLEAN TRASH command

[heguanhui]

Summary

• Fix deduplication bug in ADMIN CLEAN TRASH command where duplicate clean trash tasks could be sent to the same BE
• Add Set<Long> dedup by backend ID in both getNeedCleanedBackends() and cleanTrash() methods
• Add unit test AdminCleanTrashCommandTest covering duplicate queries, distinct queries, and clean-all scenarios

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: The ADMIN CLEAN TRASH command could send duplicate clean trash tasks to the same BE. When backendsQuery contains duplicate entries (e.g., same host:port specified twice), or when different string representations resolve to the same backend, getNeedCleanedBackends() would add the same backend multiple times. Additionally, cleanTrash() did not deduplicate by backend ID before creating tasks, so duplicate backends in the list would result in multiple CleanTrashTask objects being sent to the same BE.

Root Cause

1. In getNeedCleanedBackends(), the backendsID.remove(backendQuery) only deduplicates by the string key format (host:port). If the user provides the same backend via different string representations or duplicate entries, the same backend could be added multiple times.

2. In cleanTrash(), there was no deduplication by backend ID. The method iterated over the backends list and created a CleanTrashTask for each entry without checking for duplicates.

3. CleanTrashTask has signature -1 and is NOT added to AgentTaskQueue (only AgentTaskExecutor.submit(batchTask) is called), so the queue dedup mechanism does not apply.

Fix

• Add Set<Long> addedIds in getNeedCleanedBackends() to deduplicate by backend ID when processing backendsQuery
• Add Set<Long> addedBackendIds in cleanTrash() as a safety net to deduplicate by backend ID before creating tasks

Release note

Fixed a bug where ADMIN CLEAN TRASH could send duplicate clean tasks to the same backend when duplicate backend addresses were specified.

Check List (For Author)

• Test: Unit Test
  • AdminCleanTrashCommandTest verifies dedup with duplicate queries, distinct queries, and clean-all scenarios
• Behavior changed: No
• Does this need documentation: No

[hello-stephen]

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