修复 issue #18962 中提及的 allocateChunk() 方法的 bug .

[finscn]

static-vb-accessor 在分配 chunk时, 对可用空间的判断方式存在错误. 此pr 用来修复此 bug

相关讨论 见 #18962

Re: #

Changelog

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1009429 bytes	1009488 bytes	⚠️ +59 bytes
2D All (legacy pipeline)	2674774 bytes	2674833 bytes	⚠️ +59 bytes
2D All (new pipeline)	2765676 bytes	2765735 bytes	⚠️ +59 bytes
(2D + 3D) All	10022031 bytes	10022090 bytes	⚠️ +59 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16841090 bytes	16841149 bytes	⚠️ +59 bytes

Interface Check Report

This pull request does not change any public interfaces !