test: add test coverage and test setup for MemoryGraph class

[YuHuang0525]

Description

Add comprehensive test coverage for MemoryGraph class

• Add tests for initialization, search, delete, and get operations

• Test entity space removal functionality

• Mock Neo4j and other dependencies appropriately

Type of change

• Add test coverage for MemoryGraph class.

How Has This Been Tested?

• ✅ Unit Tests

tests/memory/test_graph_memory.py::test_memory_graph_initialization PASSED [ 20%]
tests/memory/test_graph_memory.py::test_search_with_filters PASSED         [ 40%]
tests/memory/test_graph_memory.py::test_delete_all PASSED                  [ 60%]
tests/memory/test_graph_memory.py::test_get_all PASSED                     [ 80%]
tests/memory/test_graph_memory.py::test_remove_spaces_from_entities PASSED [100%]
=============================== 5 passed in 1.27s ================================

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================== 195 passed, 6 skipped, 41 warnings in 5.20s ===================

Checklist:

• ✅ My code follows the style guidelines of this project
• ✅ I have performed a self-review of my own code
• ✅ I have commented my code, particularly in hard-to-understand areas
• ✅ I have made corresponding changes to the documentation
• ✅ My changes generate no new warnings
• ✅ I have added tests that prove my fix is effective or that my feature works
• ✅ New and existing unit tests pass locally with my changes
• ✅ Any dependent changes have been merged and published in downstream modules
• ✅ I have checked my code and corrected any misspellings

Maintainer Checklist

• ✅ Made sure Checks passed

[CLAassistant]

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