fix: point install hint at agent-governance-toolkit-core - #187

Fixes #186

Summary

AgentGovernancePlugin tells users to pip install agentmesh-platform when agentmesh can't be imported. That package is deprecated; agentmesh now ships in agent-governance-toolkit-core.

• Replace agentmesh-platform with agent-governance-toolkit-core in the plugin's error and warning messages, its docstrings, and contributing/samples/agent_governance/README.md. The replacements are in place, with no other reformatting.
• Replace test_raises_import_error_when_agentmesh_missing. It patched _setup to raise a mocked ImportError, so it only asserted on the mock's own message and never exercised the plugin's. The new test blocks the real agentmesh import through a block_agentmesh fixture and asserts on the actual message.
• Add test_fail_open_warns_with_install_hint_when_agentmesh_missing to cover the fail_open=True warning.

The fixture removes any loaded agentmesh* modules and sets sys.modules["agentmesh"] = None. Both tests therefore pass whether or not AGT is installed, even if it was imported earlier in the session.

Testing plan

• Unit tests: run the plugin tests with AGT absent, with AGT installed, and with AGT imported before the tests start. Confirm that the two new tests fail against the previous plugin code.
• E2E: construct an InMemoryRunner with the plugin in an environment without AGT, for both fail_open modes, and check the user-facing text.

pytest results

$ pytest tests/plugins/test_agent_governance_plugin.py
15 passed                                   # AGT not installed
15 passed                                   # agent-governance-toolkit-core 5.0.0 installed
15 passed                                   # agentmesh imported before pytest runs

$ pytest tests/plugins/test_agent_governance_plugin.py -k missing   # against previous plugin code
FAILED ...::test_raises_import_error_when_agentmesh_missing
FAILED ...::test_fail_open_warns_with_install_hint_when_agentmesh_missing

$ pytest tests/plugins
26 passed

$ pytest tests/unittests
79 passed

tests/plugins isn't part of the CI run (pytest tests/unittests), so the results above are from a local run on Python 3.11. tests/unittests needed greenlet installed locally: test_hitl_approval_api.py fails to collect on main without it, because the hitl extra doesn't pull it in. That's unrelated to this change.

E2E: Runner without AGT installed

import logging
from google.adk.agents import LlmAgent
from google.adk.runners import InMemoryRunner
from google.adk_community.plugins import AgentGovernancePlugin

logging.basicConfig(level=logging.WARNING, format="%(levelname)s %(name)s: %(message)s")
agent = LlmAgent(name="demo", model="gemini-2.5-flash", instruction="demo")

print("--- fail_open=False (default) ---")
try:
    InMemoryRunner(agent=agent, plugins=[AgentGovernancePlugin(policy_dir="policies")])
except ImportError as exc:
    print(f"{type(exc).__name__}: {exc}")

print("--- fail_open=True ---")
runner = InMemoryRunner(agent=agent, plugins=[AgentGovernancePlugin(policy_dir="policies", fail_open=True)])
print("Runner created with plugins:", [p.name for p in runner.plugin_manager.plugins])

$ python -u e2e_runner_missing_agt.py
--- fail_open=False (default) ---
_GovernanceUnavailableError: agent-governance-toolkit-core is required for governance enforcement. Install with: pip install agent-governance-toolkit-core
--- fail_open=True ---
WARNING google.adk_community.plugins.agent_governance_plugin: agent-governance-toolkit-core not installed; governance checks disabled. Install with: pip install agent-governance-toolkit-core
Runner created with plugins: ['agent_governance']