feat: add VaidIdentityPlugin for verified agent identity and delegation

[solara-associates]

Related to #189

Adds VaidIdentityPlugin, a thin BasePlugin in google.adk_community.plugins over vaid-adk 0.2.0 (Apache-2.0, https://github.com/solara-associates/vaid/tree/main/python/vaid-adk).

It verifies each agent's signed identity (VAID) and delegation chain before the agent runs, before it is delegated to as a sub-agent, and before every tool call, denying fail-closed whenever any of that cannot be positively confirmed. Each identity is a signed document that can only narrow as it is delegated (scope, capabilities, expiry, tenant), verified offline with a public key, with pluggable revocation. Registered once on the Runner, it covers every agent and tool in the invocation tree, including sub-agents reached through transfer_to_agent.

This mirrors the AgentGovernancePlugin shape (#141):

• src/google/adk_community/plugins/vaid_identity_plugin.py, exported from plugins/init.py the same way
• pyproject optional extra: vaid = ["vaid-adk>=0.2.0,<0.3"]
• fail-closed ImportError at import time when vaid-adk is not installed
• contributing/samples/vaid_identity/ with a README and a runnable main.py
• tests under tests/plugins/, following the existing plugin test naming

VaidIdentityPlugin performs no verification logic of its own; it delegates before_agent_callback and before_tool_callback to vaid_adk.VaidAuthPlugin.

Compatibility

vaid-adk 0.2.0 requires google-adk>=2.0,<3 and python>=3.10. This repo currently resolves google-adk 2.11.0 and requires python>=3.10, so no conflict and no environment marker is needed on the extra.

Testing plan

Unit tests under tests/plugins/test_vaid_identity_plugin.py cover: an allowed in-scope tool call, a denied out-of-scope call before the tool runs, a denied call for an agent with no registered presentation, a denied call for a tool with no entry in tool_authorizations, and the ImportError path when vaid-adk is absent. VAID documents are minted in-test with an Ed25519 key (cheap, no network), the same way vaid-adk's own tests do.

Pytest summary:

tests/plugins/test_vaid_identity_plugin.py::test_plugin_name PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_agent_callback_allows_registered_identity PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_agent_callback_denies_unregistered_agent PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_tool_callback_allows_in_scope_call PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_tool_callback_denies_out_of_scope_call PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_tool_callback_denies_unregistered_agent PASSED
tests/plugins/test_vaid_identity_plugin.py::test_before_tool_callback_denies_unmapped_tool_never_default_allow PASSED
tests/plugins/test_vaid_identity_plugin.py::test_raises_import_error_when_vaid_adk_missing PASSED

8 passed in 1.08s

Full suite (unrelated to this change) remains green: 82 passed, 14 skipped.

Runner E2E

contributing/samples/vaid_identity/main.py runs through a real ADK Runner with no model or API key (the two agents are plain BaseAgent subclasses, so the sample exercises the genuine ADK plugin_manager extension points rather than a mock of them). A coordinator delegates to a worker holding an attenuated VAID; the worker's in-scope tool call is allowed and its out-of-scope call is denied before the tool runs.

Command: python contributing/samples/vaid_identity/main.py

Console output:

[coordinator] delegating to worker
[worker] lookup_order: ALLOWED
[worker] issue_refund: DENIED (out_of_scope)

Install: pip install google-adk-community[vaid]