[ISSUE #12884] Fix Flaky Behavior in NewMetadataSelector Tests

[DaniellaHubble]

In response to ISSUE #12884

What is the purpose of the change

The purpose is to fix a flakiness issue in the com.alibaba.nacos.client.naming.selector.NamingSelectorFactoryTest.testNewMetadataSelectorand testNewMetadataSelector2 tests, caused by the non-deterministic iteration order of HashMap, which led to inconsistent test outcomes when metadata key-value pairs were accessed.

You can see this issue because testNewMetadataSelector and testNewMetadataSelector2 passed when I ran them normally using the command: mvn -pl client test -Dtest=com.alibaba.nacos.client.naming.selector.NamingSelectorFactoryTest#testNewMetadataSelector but were failing when ran under NonDex, a testing tool that identifies flaky tests caused by non-deterministic behaviors. To run with NonDex, I used the command: mvn -pl client edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=com.alibaba.nacos.client.naming.selector.NamingSelectorFactoryTest#testNewMetadataSelector

Brief changelog

• Replaced HashMap with LinkedHashMap in testNewMetadataSelector to ensure deterministic iteration order for metadata matching.

• Explicitly set LinkedHashMap for instance metadata to preserve insertion order during matching.

Verifying this change

To verify this change, I ran the following to execute the tests with NonDex for 10 iterations: mvn -pl client edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=com.alibaba.nacos.client.naming.selector.NamingSelectorFactoryTest#testNewMetadataSelector -DnondexRuns=10
and for the second test I ran:
mvn -pl client edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=com.alibaba.nacos.client.naming.selector.NamingSelectorFactoryTest#testNewMetadataSelector2 -DnondexRuns=10
These commands ensured each test ran 10 times under NonDex. Both testNewMetadataSelector and testNewMetadataSelector2 passed consistently, confirming the absence of flakiness.

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[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

All modified and coverable lines are covered by tests ✅

Project coverage is 72.26%. Comparing base (c5e5a82) to head (34ecdda).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #12885      +/-   ##
=============================================
- Coverage      72.27%   72.26%   -0.02%     
  Complexity      9827     9827              
=============================================
  Files           1287     1287              
  Lines          41445    41445              
  Branches        4373     4373              
=============================================
- Hits           29956    29949       -7     
- Misses          9387     9395       +8     
+ Partials        2102     2101       -1     

see 2 files with indirect coverage changes

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