Add tests for plugin failure modes (timeouts, bad JSON, missing evidence)

[PAXECT-Interface]

Summary

Add more tests to cover plugin failure modes in Open Agentic 2.0.
The goal is to make sure the orchestrator behaves correctly when plugins fail
(e.g. timeouts, bad JSON, or missing evidence).

What to test

Where possible, add or extend tests in tests/test_agentic2.py (or a new test
file if that makes more sense) to cover cases like:

• a plugin call that times out (e.g. subprocess / HTTP timeout),
• a plugin that returns invalid JSON or non-JSON output,
• a plugin that returns a response without evidence or with too-low coverage.

When this is done

• There are automated tests that simulate plugin failures (timeout / bad JSON /
  missing or low-quality evidence).
• The orchestrator:
  • logs these failures in the audit chain, and
  • either blocks / abstains as expected (fail-closed), instead of silently
    returning ok=True.
• Running pytest -q -vv still passes.