refactor: use 1 for vnode count of singletons for backward compatibility

[BugenZhao]

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What's changed and what's your intention?

This is a progress towards #15900.

Try to address #18515 (comment), i.e., use 1 for vnode count of singletons when it comes to backward compatibility.

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[BugenZhao]

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• refactor: use 1 for vnode count of singletons for backward compatibility #18753 👈
• fix(meta): persist correct vnode count for catalogs #18759
• feat(frontend): show job's max parallelism in system tables #18672
• refactor(meta): persist job-level max parallelism & check when ALTER .. SET PARALLELISM #18740
• fix(meta): fix query for running_fragment_parallelisms in postgres backend #18743
• main

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[BugenZhao]

This reveals a severe bug that we persist and notify about the internal tables before the vnode counts are correctly resolved, leading to inconsistency between the stream nodes and the catalog.

[BugenZhao]

This reveals a severe bug

Will first fix in #18759