Support shared mbvar

[chenBright]

What problem does this PR solve?

Issue Number: resolve #3069

Problem Summary:

MultiDimension get_stats returns a raw pointer, making it unsafe to delete bvar via delete_stats.

What is changed and the side effects?

Changed:

MultiDimension<BvarType, KeyType, true> get_stats supports returning shared bvar(std::shared_ptr), so bvar can be safely deleted via delete_stats.

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull Request Overview

This PR adds support for shared (thread-safe) bvar management in MultiDimension by introducing a template parameter Shared that controls whether get_stats returns a raw pointer or std::shared_ptr. When Shared=true, the shared pointer approach prevents use-after-free issues when deleting bvars concurrently.

Key changes:

• Added Shared template parameter (defaulting to false for backward compatibility) to MultiDimension
• Modified return types to use std::shared_ptr when Shared=true, raw pointers when Shared=false
• Moved test code (MyStringView class and TestLabels function) from bvar_mvariable_unittest.cpp to bvar_multi_dimension_unittest.cpp and added new concurrency test

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
test/bvar_mvariable_unittest.cpp	Removed MyStringView class definition, TestLabels function, and labels test; cleaned up unused variable
test/bvar_multi_dimension_unittest.cpp	Added MyStringView class, TestLabels function, labels test, and new thread-safety test for shared bvar
src/bvar/multi_dimension_inl.h	Updated all method signatures and implementations to support Shared template parameter; replaced nullptr with NULL for consistency
src/bvar/multi_dimension.h	Added Shared template parameter, conditional typedef for value_ptr_type, and helper methods for creating/deleting values based on Shared flag

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[wwbmmm]

Note: The shared mbvar may be less performant than the non-shared version.

[chenBright]

Note: The shared mbvar may be less performant than the non-shared version.

Documentation and comments have been updated.

[wwbmmm]

LGTM