[fix](case) test_admin_compact_table: count distinct tablets for multi-replica clusters

[shuke987]

Problem

On a multi-replica test cluster (e.g. force_olap_table_replication_num=3), SHOW TABLETS returns one row per (tablet, replica). A table created with BUCKETS 1 therefore yields 3 rows, so assertEquals(1, tablets.size()) fails (expected 1, got 3). The assertion implicitly assumes a single replica.

Fix

Assert on the distinct tablet count: tablets.collect { it.TabletId }.unique().size(). The subsequent code already picks one replica via tablets[0].

Verification

Ran the suite on a branch-4.1 local cluster (3 FE + 4 BE, force-3 replicas) — passes.

🤖 Generated with Claude Code

[hello-stephen]

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[shuke987]

run buildall