fix(event): should log reason string instead of id in EventListener::OnFlushCompleted

[Ryan4253]

Closes #3008

Description of Changes:

EventListener::OnFlushCompleted currently logs the enum directly in the reason field. This commit switches to logging the reason string instead which gives better ergonomics when reading logs.

Test Plan:

Unit Test

----------] Global test environment tear-down
[==========] 430 tests from 69 test suites ran. (18988 ms total)
[  PASSED  ] 429 tests.
[  SKIPPED ] 1 test, listed below:
[  SKIPPED ] UseBitmap/RedisBitmapTest.BitfieldStringGetSetTest/0

Logs:

Before:

[2025-06-03T14:30:37.476414-04:00][I][event_listener.cc:137] [event_listener/flush_completed] column family: metadata, thread_id: 87921, job_id: 11, file: [dir]/kvrocks/db/000477.sst, reason: 6, is_write_slowdown: no, is_write_stall: no, largest seqno: 15602, smallest seqno: 15256

After:

[2025-06-03T10:45:33.144102-04:00][I][event_listener.cc:145] [event_listener/flush_completed] column family: metadata, thread_id: 121487, job_id: 6, file: [dir]/kvrocks/db/000110.sst, reason: Write Buffer Full, is_write_slowdown: no, is_write_stall: no, largest seqno: 4096, smallest seqno: 3754

[mapleFU]

Seems failed ut unrelated, would retrigger ci before merging

--- FAIL: TestBloomInfo (10.12s)
    --- FAIL: TestBloomInfo/MAdd_scaling_Test (0.02s)
        bloom_test.go:243: 
            	Error Trace:	/Users/runner/work/kvrocks/kvrocks/tests/gocase/unit/type/bloom/bloom_test.go:243
            	Error:      	Not equal: 
            	            	expected: []interface {}{"Capacity", 210, "Size", 896, "Number of filters", 3, "Number of items inserted", 91, "Expansion rate", 2}
            	            	actual  : []interface {}{"Capacity", 90, "Size", 384, "Number of filters", 2, "Number of items inserted", 90, "Expansion rate", 2}

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud