feat: Add MemMachine Integration Guide for langchain.

[tianAndrew]

Purpose of the change

Add MemMachine Integration Guide for langchain.

Description

Add MemMachine Integration Guide for langchain. Use MemMachine MCP.

Fixes/Closes

Fixes #783 #784

Type of change

[Please delete options that are not relevant.]

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g., code style improvements, linting)
• Documentation update
• Project Maintenance (updates to build scripts, CI, etc., that do not affect the main project)
• Security (improves security without changing functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

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• Unit Test
• Integration Test
• End-to-end Test
• Test Script (please provide)
• Manual verification (list step-by-step instructions)

Test Results: [Attach logs, screenshots, or relevant output]

(base) andrew@andrewdeMBP MemMachine % python integrations/langchain/test_memory.py
============================================================
LangChain + MemMachine Integration Tests
============================================================

Checking MemMachine server at http://localhost:8080...
✓ MemMachine server is available at http://localhost:8080

✓ LangChain integration is available

Running basic workflow test...
------------------------------------------------------------
Test IDs: test_langchain_org_9114c157/test_langchain_project_c72594df

   Saving context: 'My name is Bob and I love Python'
   ✓ Context saved
   Waiting for indexing...
   Loading memory variables...
   History length: 66
   History preview: Human: My name is Bob and I love Python
AI: Nice to meet you, Bob!
   Loading memory for preferences...
   History: human: my name is bob and i love python
ai: nice to meet you, bob!
human: i also like machine learning
ai: that's interesting!

✓ Basic workflow test passed!

============================================================
All tests passed!
============================================================

To run full test suite with pytest:
  pytest integrations/langchain/test_memory.py -v -m integration

Note: Some tests may require additional dependencies (e.g., langchain)

Checklist

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• I have signed the commit(s) within this pull request
• My code follows the style guidelines of this project (See STYLE_GUIDE.md)
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added unit 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

• Confirmed all checks passed
• Contributor has signed the commit(s)
• Reviewed the code
• Run, Tested, and Verified the change(s) work as expected

Screenshots/Gifs

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Further comments

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