AgentGovernancePlugin: install hint points at deprecated agentmesh-platform #186

Describe the bug
AgentGovernancePlugin tells users to pip install agentmesh-platform when agentmesh can't be imported (error/warning messages and docstrings in src/google/adk_community/plugins/agent_governance_plugin.py, plus contributing/samples/agent_governance/README.md). That package is deprecated; the Agent Governance Toolkit now ships agentmesh in agent-governance-toolkit-core.

To Reproduce

1. In a fresh environment: pip install google-adk-community (without AGT).
2. Construct AgentGovernancePlugin(policy_dir=...).
3. The error says Install with: pip install agentmesh-platform.

Expected behavior
The hint should name agent-governance-toolkit-core.

Proposed fix
Update the messages, docstrings and sample README, and add a unit test that makes the agentmesh import actually fail and asserts on the message. The existing test patches _setup, so the real message is never exercised. I'm happy to send the PR.

Related: #185