fix(cli): read Workflow.graph when drawing the dev UI graph

[khairulislam]

Link to Issue or Description of Change

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

• Closes: Dev UI graph fails for Workflow root agents: build_graph reads agent._graph, which doesn't exist #7528

Problem:
build_graph() reads agent._graph for Workflow agents, but Workflow only has the public field graph. Any Workflow root agent makes GET /dev/apps/{app_name}/graph return 500, so the dev UI cannot draw the workflow:

File "google/adk/cli/agent_graph.py", line 200, in build_cluster
    elif isinstance(agent, Workflow) and agent._graph is not None:
AttributeError: 'Workflow' object has no attribute '_graph'. Did you mean: 'graph'?

Running the workflow is not affected. Present in v2.11.0, v2.12.0 and main.

Solution:
Use agent.graph (3 occurrences). No other change.

Testing Plan

Unit Tests:

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

Added test_build_graph_workflow_draws_nodes_and_edges_from_its_graph to tests/unittests/cli/test_agent_graph.py: a Workflow with an agent, function nodes, a retry back-edge and a DEFAULT_ROUTE edge. It fails on main with the AttributeError above and passes with this change.

$ pytest tests/unittests/cli/test_agent_graph.py -q
10 passed

Manual End-to-End (E2E) Tests:

adk web with a Workflow root agent (plan -> review -> write -> review, each review looping back on a retry route): GET /dev/apps/<app>/graph returns 500 on main. With this change it returns 200 and the DOT source includes the nodes and the retry back-edges (review_outline -> BlogPlanner, review_post -> BlogWriter).

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

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[khairulislam]

Test output from pytest tests/unittests/cli -n 8 -q : test_output.txt