feat: implement powermem langchain middleware

[kazmiller0]

• before_agent/abefore_agent search PowerMem using the latest user message and inject retrieved memories as a SystemMessage
• after_agent writes back the user/assistant exchange via memory.add, gated on save_interactions
• explicit user_id threaded through all PowerMem calls
• fail-open: search failures are logged and swallowed so the agent still runs
• async path delegates to the sync implementation since PowerMem's Memory client is sync and LangGraph's RunnableCallable supports sync-only hooks under async execution

Summary

Solution Description

[CLAassistant]

All committers have signed the CLA.

[LINxiansheng]

LangChain Contract Score

LangChain Contract Score
========================
score: 100.00 / 100
suites: 2
rule: A weighted capability receives credit only when it passes on every configured Python suite and all declared prerequisite capabilities also receive credit. Gate tests are reported but do not add points.

Public import contract: PASS (gate)
  py311: passed
  py312: passed
Retrieve memory before model call: PASS (35/35)
  py311: passed
  py312: passed
Persist interaction after agent run: PASS (25/25)
  py311: passed
  py312: passed
Disable interaction persistence: PASS (10/10)
  py311: passed
  py312: passed
Fail-open on search error: PASS (15/15)
  py311: passed
  py312: passed
Async memory injection: PASS (15/15)
  py311: passed
  py312: passed

LangChain Contract Report

LangChain Contract Report
=========================
score: 100.00 / 100
rule: A weighted capability receives credit only when it passes on every configured Python suite and all declared prerequisite capabilities also receive credit. Gate tests are reported but do not add points.

Suite Summary
-------------
py311: passed=6/6, failures=0, errors=0, skipped=0
py312: passed=6/6, failures=0, errors=0, skipped=0

Gate Checks
-----------
Public import contract [gate] PASS
  - py311: passed
  - py312: passed

Weighted Breakdown
------------------
Retrieve memory before model call [35/35] PASS
  - py311: passed
  - py312: passed
Persist interaction after agent run [25/25] PASS
  - py311: passed
  - py312: passed
Disable interaction persistence [10/10] PASS
  - py311: passed
  - py312: passed
Fail-open on search error [15/15] PASS
  - py311: passed
  - py312: passed
Async memory injection [15/15] PASS
  - py311: passed
  - py312: passed

Workflow run

[Teingi]

Thank you for participating in the VLDB Summer School. Now that the program has concluded, we’re closing this PR as part of the event wrap-up. We appreciate the time and effort you put into your contribution.