[Bug] Fix TOCTOU race condition and unsafe lazy init in CacheItem.initConfigGrayIfEmpty()

[LiyunZhang10]

Issue Description

In CacheItem.java, configCacheGray was lazily initialized using a non-thread-safe HashMap and without synchronization (missing Double-Checked Locking). When multiple threads call initConfigGrayIfEmpty(grayName) concurrently, multiple HashMaps could be created, overwriting each other and causing memory leak and data loss.

Additionally, the method used a check-then-act pattern (containsKey + put) which is not atomic. In high-concurrency gray release configurations, this could lead to multiple createConfigCacheGray() executions for the same grayName, and throwing ConcurrentModificationException due to the non-thread-safe HashMap implementation.

Proposed Changes

• Added Double-Checked Locking (DCL) around the lazy initialization of configCacheGray (which is already volatile).
• Changed configCacheGray's underlying implementation from HashMap to ConcurrentHashMap to ensure thread safety during concurrent structural modifications.
• Replaced the containsKey + put pattern with computeIfAbsent() to guarantee that ConfigCacheFactoryDelegate.createConfigCacheGray() is executed exactly once per grayName, effectively eliminating the TOCTOU hazard and preventing corrupted pointers that could lead to NPE in the routing layer.

General Checklist

• Logic change explained clearly
• No changes to public API surface
• No additional dependencies introduced

[github-actions]

Thanks for your this PR. 🙏
Please check again for your PR changes whether contains any usage/api/configuration change such as Add new API , Add new configuration, Change default value of configuration.
If so, please add or update documents(markdown type) in docs/next/ for repository nacos-group/nacos-group.github.io

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感谢您提交的PR。 🙏
请再次查看您的PR内容，确认是否包含任何使用方式/API/配置参数的变更，如：新增API、新增配置参数、修改默认配置等操作。
如果是，请确保在提交之前，在仓库nacos-group/nacos-group.github.io中的docs/next/目录下添加或更新文档（markdown格式）。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...m/alibaba/nacos/config/server/model/CacheItem.java	83.33%	0 Missing and 1 partial ⚠️

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