feat: support per-fragment vnode count

[BugenZhao]

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What's changed and what's your intention?

This is a progress towards #15900.

In previous PRs, we mainly focused on refactoring codes from different modules to avoid directly referencing the global constant of VirtualNode::COUNT, but infer it from the context or directly accept it as a parameter.

In this PR, we introduce the vnode_count field for Table and StreamFragmentGraph, update the logic for its maintenance. Many TODOs of directly using VirtualNode::COUNT are fixed by reading the values from Fragment or TableDesc.

Note that there's still no user-facing changes in this PR, but we are now quite close to that. However, there are still some unaddressed TODOs that may require changes in behavior. We leave this in future PRs.

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.

[BugenZhao]

• feat: user-facing part of variable vnode count #18515
• feat(streaming): support up to 16-bit vnode count in row id gen #18529
• feat: support per-fragment vnode count #18444 👈
• feat(storage): variable vnode count support #18415
• refactor(frontend): extract filling fields in fragment graph #18466
• main

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