fix(a2a): Support to_a2a(Workflow) and reject non-agent root nodes

[sokoliva]

Link to Issue

• Related: to_a2a(Workflow) fails when Workflow contains LlmAgent with input_schema/output_schema (ADK 2.0.0b1) #5487

Problem:
to_a2a(workflow) fails because AgentCardBuilder requires field sub_agents which Workflow does not have.

Solution:

• Make to_a2a() and AgentCardBuilder accept Workflow (the v2 graph orchestrator) as a root, not just BaseAgent. Previously crashed withRuntimeError: 'Workflow' object has no attribute 'sub_agents'.
• Tighten the public type contract to BaseAgent | Workflow and reject other BaseNode subtypes (e.g. FunctionNode, JoinNode) at call time with TypeError. They previously produced a degenerate "custom agent" card silently.

Testing Plan

tests/unittests/a2a/utils/test_agent_card_builder.py — 9 new tests:

• test_get_agent_type_workflow — returns 'graph_workflow' for the new v2 Workflow.
• test_get_agent_skill_name_workflow — returns 'workflow' for Workflow.
• test_init_rejects_function_node — AgentCardBuilder(agent=FunctionNode(...)) raises TypeError (regression coverage for the runtime guard).
• test_init_rejects_arbitrary_object — AgentCardBuilder(agent="...") raises TypeError.
• test_build_succeeds_for_llm_agent — regression coverage that the original BaseAgent path still works after the type narrowing.
• test_build_succeeds_for_workflow_with_llm_agent_node — exact OP repro shape, end-to-end through build().
• test_build_succeeds_for_workflow_with_output_schema_node — covers the output_schema shape mentioned in the issue.
• test_build_succeeds_for_empty_workflow — degenerate but valid case (no edges).
• test_get_workflow_description_workflow_with_nodes — verifies graph nodes appear in the description string.
• test_get_workflow_description_empty_workflow — returns None when no nodes.
  tests/unittests/a2a/utils/test_agent_to_a2a.py — 2 new tests, 1 rewritten:
• test_to_a2a_succeeds_for_workflow (new) — end-to-end through the actual Starlette lifespan (the exact code path that crashed in the OP's repro).
• test_to_a2a_rejects_function_node (new) — to_a2a(FunctionNode(...)) raises TypeError at call time.
• test_to_a2a_rejects_non_agent_non_workflow (rewrote existing test_to_a2a_with_invalid_agent_type) — now asserts TypeError raised eagerly at to_a2a() call time instead of AttributeError raised lazily during request handling. This is a deliberate behavior change, not a regression: the old test encoded buggy lazy-failure UX where misuse only surfaced when a client hit the endpoint.

Unit Tests:

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

Please include a summary of passed pytest results.
ran uv run pytest tests/unittests

6275 passed, 14 skipped, 25 xfailed, 10 xpassed, 2532 warnings

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.