database "system" is dropped when api_version=2

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

{"log":"[TiDBSchemaSyncer.h:170] [\"End sync schema, version has been updated to 0(latest diff is empty)\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}
{"log":"[SchemaBuilder.cpp:1415] [\"Dropped all schemas.\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}
{"log":"[SchemaBuilder.cpp:979] [\"Tombstoned database system\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}
{"log":"[SchemaBuilder.cpp:958] [\"Tombstoning database system\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}
{"log":"[SchemaBuilder.cpp:1385] [\"Dropping all schemas.\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}
{"log":"[TiDBSchemaSyncer.h:135] [\"Start to drop schemas. schema version key not exists, keyspace should be deleted\"] [source=\"keyspace=4294967295\"] [thread_id=27]","container":"data0","pod":"tc-tiflash-1","level":"INFO","namespace":""}

4. What is your TiFlash version? (Required)

master

Start by reading SchemaBuilder.cpp around lines 958, 979, 1385, 1415 and TiDBSchemaSyncer.h around lines 135 and 170, then establish a reproducible api_version=2 case from the reported logs. Done means the system database is no longer dropped during schema synchronization, with the relevant behavior verified against a regression test if the existing test structure identifies one.