[Test] Add unit tests for untested components in impl module

[LHT129]

Description

Add unit tests for untested components in the src/impl/ module to improve test coverage and regression protection.

Background

The src/impl/ module is the core implementation utility library of vsag, containing allocators, bitsets, BLAS wrappers, filters, heaps, loggers, searchers, thread pools, and transformers. While most components have good test coverage, approximately 12 source files lack unit tests.

Requirements

• Add test for BlasFunction (Saxpy, Sscal, Sgemv, Sgemm)
• Add test for CombinedFilter (AND combination logic)
• Add test for InnerIdWrapperFilter (ID mapping correctness)
• Add test for ComputableBitset (factory methods)
• Add test for AllocatorWrapper (allocate, deallocate, construct, destroy)
• Add test for DefaultThreadPool (Enqueue, WaitUntilEmpty, SetPoolSize)
• Add test for MRLETransformer (Transform for L2/Cosine metrics)

Acceptance Criteria

• All new tests pass
• Code compiles successfully
• Code follows project style guidelines

Related

• Original task: agent-hive/tasks/2026-03-13-补充-impl-模块未测试组件测试.md