fix(streaming): fix compatibility upgrading from v2.1.0 for singleton vnode count

[BugenZhao]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

In #18753, we mentioned that...

For SQL backend, correctly perform the migration to fill 1 for the vnode_count field of singletons.

However, consider the singleton internal table of the Source executor, of which maybe_vnode_count in its protobuf representation is set to the value of hash-distributed Source fragment if users upgrade from v2.1. Our migration script on SQL models can't cover such case, which break the assertions introduced later in #19267.

This should address the backward-compat test failure in main-cron CI these days.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.