fix(agent_registry): source MCP destination ID from RuntimeReference.uri

[chelsealong]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: AgentRegistry.get_mcp_toolset() reads the wrong field for gcp.mcp.server.destination.id — breaks Agent Platform Topology view #7196

Problem:
AgentRegistry.get_mcp_toolset() stamps the gcp.mcp.server.destination.id
span attribute (used by the Agent Platform Topology view to draw a
connection from an agent to the MCP server it calls) from the Agent
Registry API's mcpServerId field, a urn:mcp:... value. The Topology
view's connection matcher resolves against the App Hub resource-name URI
form instead (//agentregistry.googleapis.com/projects/<number>/locations/ <region>/services/<name>), which the same API response already returns,
unused, under
attributes["agentregistry.googleapis.com/system/RuntimeReference"]["uri"].
Because the two identifier forms never match, no topology connection is
ever drawn for an MCP server reached through AgentRegistry /
AgentRegistrySingleMcpToolset, regardless of correct registration.

Solution:
In get_mcp_toolset(), source destination_resource_id from
attributes["agentregistry.googleapis.com/system/RuntimeReference"]["uri"]
when present, falling back to mcpServerId only if that attribute is
absent. mcpServerId itself is left untouched everywhere else in the file
(e.g. _resolve_auth_provider_scheme, which matches it against IAM
binding targets and still expects the urn form) — this is a one-field,
single-call-site change.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added test_get_mcp_toolset_prefers_runtime_reference_uri to
tests/unittests/integrations/agent_registry/test_agent_registry.py,
which mocks an MCP server response containing both mcpServerId and
attributes["agentregistry.googleapis.com/system/RuntimeReference"]["uri"]
and asserts the tool's custom_metadata[GCP_MCP_SERVER_DESTINATION_ID]
is the RuntimeReference.uri value, not mcpServerId.

Confirmed the new test fails without the fix (reverting only
agent_registry.py via git stash) with:

AssertionError: assert 'urn:mcp:goog...obal:bigquery' == '//agentregis...ices/bigquery'

- //agentregistry.googleapis.com/projects/1234/locations/global/services/bigquery
+ urn:mcp:googleapis.com:projects:1234:locations:global:bigquery

Full suite for the touched module, with the fix applied:

$ pytest tests/unittests/integrations/agent_registry/ -q
55 passed, 13 warnings in 2.73s

pre-commit run --files src/google/adk/integrations/agent_registry/agent_registry.py tests/unittests/integrations/agent_registry/test_agent_registry.py
passes (ruff, isort, pyink, addlicense, compliance checks, codespell).

Manual End-to-End (E2E) Tests:

Not performed — this requires a live Google Cloud Agent Registry
deployment with a registered MCP server and OTLP export to
telemetry.googleapis.com, which is unavailable in this environment.
Covered instead by the unit test above, which reproduces the exact
API response shape from the issue report (both mcpServerId and
RuntimeReference.uri present) and asserts the observable output
(custom_metadata[GCP_MCP_SERVER_DESTINATION_ID]).

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end (see note above).

Additional context

This change was written with AI assistance (Claude Code), with all
changes reviewed and verified by the human account opening this PR.

Fixes #7196