fix(workflow): Skip task dispatch on progressive-SSE partial chunks

[a2105z]

Link to Issue or Description of Change

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

• Closes: chat wrapper (run_llm_agent_as_node) extracts task-delegation FCs from partial=True progressive-SSE chunks and breaks — the FC event is never persisted, orphaning the synthesized task FR and permanently poisoning the session #6583

Problem:
Under RunConfig(streaming_mode=StreamingMode.SSE) with progressive SSE (default-on), intermediate model chunks are marked partial=True, including chunks that already name a task-delegation function call. The chat wrapper's _extract_task_delegation_fcs had no partial gate, so it dispatched the child task and breaks out of run_async before the Runner-persisted non-partial aggregate was yielded.

Effects:

1. The matching task FC never lands in session history, while the synthesized task FR does → orphaned FR (hard raise on older ADK; prune/warn on current main).
2. Dispatch can fire from a truncated/empty partial args chunk instead of the complete aggregate.

Solution:
Ignore task-delegation FCs when event.partial is true in _extract_task_delegation_fcs, matching the existing partial gate in process_llm_agent_output in the same module. Dispatch + break then wait for the non-partial aggregate the Runner actually persists (with complete streamed args).

This matches the approach discussed and verified on #6583.

Testing Plan

Unit Tests:

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

• All unit tests pass locally.

• test_extract_task_delegation_fcs_skips_partial_events — unit gate

• test_chat_root_dispatches_task_fc_only_from_non_partial_sse_chunk — e2e under StreamingMode.SSE (complete args, single dispatch, matched FC/FR ids, usable next turn)

• Confirmed both tests fail without the gate (orphaned FR / prune path)

• pytest tests/unittests/workflow/test_llm_agent_as_node.py tests/unittests/workflow/test_task_api_e2e.py → 60 passed, 8 skipped, 1 xfailed

Manual End-to-End (E2E) Tests:

Covered by the progressive-SSE e2e regression above (custom LLM emits partial incomplete FC then non-partial aggregate with full args).

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.

Additional context

@llalitkumarrr asked for this PR on #6583 after verifying the partial-gate approach against the reproduction.

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