refactor(pipeline): collapse invoke() to single return path

[miguelgfierro]

Addresses finding #5 from the simplifier review of #232.

StatePipeline.invoke had seven return await self._finalize_run(StatePipelineResult(...), pipeline_span, pipeline_start_time, run_id) call sites — each passing the same three captured locals. This collapses them into one try/finally:

• Each early-termination path now assigns result = StatePipelineResult(...) and breaks.
• The finally block ends the pipeline span and emits on_pipeline_complete exactly once.
• _finalize_run helper removed.

Net −32 LOC in state_pipeline.py (144 insertions, 176 deletions).

Behavior

Unchanged:

• The early "resume finds run already complete" return still bypasses the observability boundary (same as before — it returns before on_pipeline_start is emitted).
• All exception paths still emit on_pipeline_complete with the correct success value.
• Node-level audit, checkpoint, and span lifecycle are untouched.

Verification

• pytest tests/unit/pipeline/ → 142 passed.
• ruff check + ruff format --check clean.