Add Dify compatibility contract checks

[awxxxxxx]

Summary

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

This PR covers the plugin-upgrade side. The main mem9 compatibility workflow can now checkout this repo with plugin_ref and run Dify contract checks as part of the cross-agent matrix.

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 PR adds:

python3 compat agent dify --lane plugin-contract --plugin-ref <this-repo-branch-or-tag>

Dify does not currently have a real headless host smoke lane in the same way as OpenClaw/Hermes/Claude/OpenCode/Codex, so the blocking gate is the plugin contract suite. The main workflow still includes Dify in agent=all plugin/full matrices.

GitHub Action usage from mem9 main

Dify plugin upgrade check:

agent: dify
lane: plugin-contract
plugin_ref: <this PR branch/tag/SHA>
host_ref: <empty>
plugin_source: manifest

Full matrix check:

agent: all
lane: full
host_channel: stable
plugin_source: local

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.