bugfix: rollback shared prefix blocks on allocate failure.

[RobbieLeung]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces rollback logic in BlockManagerPool::allocate to handle allocation failures by releasing shared prefix blocks and embedding IDs. It also adds a unit test to verify this functionality. Review feedback highlights a resource leak caused by the incorrect ordering of reset operations and a memory leak in the test suite resulting from the use of release() instead of reset().

[gemini-code-assist]

Using engine.release() here causes a memory leak of the FakeEngine object. More importantly, it bypasses the BlockManagerImpl destructor which contains a critical safety check (CHECK_EQ(num_free_blocks_, free_blocks_.size() - 1)) to ensure all blocks have been correctly freed. This check is essential for verifying that the rollback logic being tested actually works as expected and doesn't leak blocks. Use engine.reset() or simply let it go out of scope instead.

Suggested change

	(void)engine.release();
	engine.reset();