Add Dify compatibility contract checks

[awxxxxxx]

Summary

Adds repo-local contract tests and CI for the mem9 Dify plugin.

What changed

• Adds .github/workflows/compat-checks.yml.
• Adds tests/test_contract.py with a stubbed Dify runtime so tests do not need a live Dify server.
• Verifies provider credential validation and tool request contracts.

What the tests cover

• Single-space credential validation sends X-API-Key and X-Mnemo-Agent-Id.
• Single-space mode requires an API key.
• Multi-space search returns a clear error when node-level API key is missing.
• memory_search sends expected headers and params, including session_id, scanAll, and upstream limit * 3.
• memory_store sends smart ingest payload with messages[], agent_id, mode=smart, and session_id.

How this relates to mem9 main

The main mem9 repo now has a cross-agent compatibility workflow that can checkout this repo and run Dify contract checks for agent upgrade validation. This PR gives the Dify plugin repo first-party CI coverage as well.

Verified locally

python3 -m venv /tmp/mem9-dify-ci-venv
/tmp/mem9-dify-ci-venv/bin/python -m pip install --disable-pip-version-check pytest requests
/tmp/mem9-dify-ci-venv/bin/python -m pytest -q tests/test_contract.py

Result: 5 passed.