fix(meta): fix query for running_fragment_parallelisms in postgres backend

[BugenZhao]

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What's changed and what's your intention?

The previous query in #18515 works for SQLite but will be rejected by PostgreSQL. Rewritten with sea-query. Tested locally.

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

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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.

[BugenZhao]

• refactor: use 1 for vnode count of singletons #18753
• fix(meta): persist correct vnode count for table catalogs in sql backend #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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