Collect compaction_low_thread_score in compaction diagnostics

[wayyoungboy]

Summary

Fixes #1237.

Adds compaction_low_thread_score collection to both compaction diagnostics paths:

• observer.compaction gather now collects the parameter from oceanbase.__all_virtual_tenant_parameter_info, ordered by tenant and server scope, so all tenant/server values are included.
• rca major_hold now records and saves compaction_low_thread_score at the start of the diagnosis.

The RCA records value 0 as OceanBase using the internal default thread score instead of treating it as an error.

Verification

PYTHONPATH=. python -m pytest test/rca/test_major_hold_compaction_thread_score.py -q
black --check -S -l 256 plugins/rca/major_hold.py test/rca/test_major_hold_compaction_thread_score.py
PYTHONPATH=. python -m pytest test/common/test_scene.py -q
PYTHONPATH=. python -m pytest test/rca/test_major_hold_compaction_thread_score.py test/common/test_scene.py -q
git diff --check

Results:

• test_major_hold_compaction_thread_score.py: 2 passed
• black check: passed
• test_scene.py: 17 passed
• combined: 19 passed
• diff check: passed