fix(cdc): skip mysql no-pk tables in initial snapshot unless chunk key configured

[Cq-study]

Proposed changes

Issue Number: close #641

Problem Summary:

In MySQL-to-Doris full-database CDC sync (mysql-sync-database), tables without primary keys were not filtered automatically.
When startup mode is initial (incremental snapshot phase), these no-PK tables can fail split-based snapshot reading and cause repeated Flink job restarts.

This PR adds a guard in MySQL schema discovery to improve stability by default.

What is changed

1. In MysqlDatabaseSync#getSchemaList, skip tables that meet all of the following:

• table has no primary key
• startup mode is initial (or empty, treated as initial)
• no chunk key is configured for that table

2. Keep no-PK tables syncable when chunk key is explicitly configured via:

• scan.incremental.snapshot.chunk.key.column (table-level mapping)

3. Add warning logs for skipped tables with clear guidance to configure chunk key.

4. Add fail-fast message when all matched tables are skipped:

• throw explicit exception describing why no table is left for synchronization.

5. Add unit tests:

• MysqlDatabaseSyncTest
  • testSkipTableWithoutPrimaryKeyInInitialSnapshot
  • testDoNotSkipTableWithoutPrimaryKeyWhenChunkKeyConfigured
  • testDoNotSkipTableWithoutPrimaryKeyForNonInitialStartup

Why

This avoids default job instability in common real-world schemas where some source tables do not have primary keys, while preserving an explicit opt-in path (chunk key) for required no-PK tables.

Checklist(Required)

1. Does it affect the original behavior: Yes (only for no-PK tables in initial snapshot mode without chunk key; behavior becomes safer by default)
2. Has unit tests been added: Yes
3. Has document been added or modified: No
4. Does it need to update dependencies: No
5. Are there any changes that cannot be rolled back: No

Further comments

This change is intentionally scoped to MySQL CDC database sync path and does not alter sink logic or other source connectors.

[JNSimba]

This is a behavior change; it's recommended to add a configuration instead of enabling it by default.