test: patch dify-agent result key cache

[vicksiyi]

Summary

• Update dify-agent compatibility tests to patch the lazy result-key cache introduced by the client change.
• Preserve the existing coverage for both current result payloads and legacy part payloads.

Tests

• uv run --extra server --group dev python -m compileall src/dify_agent/client/_client.py tests/local/dify_agent/client/test_client.py
• direct smoke check for _function_tool_result_payload_key_cache overrides

Notes

• Target pytest collection is blocked locally by a Python segfault in pytest capture initialization while loading black extension modules, before the selected tests run.

Fixes #37699