Refactor IntermediateStep to ATIFConverter

[AnuradhaKaruppiah]

Description

What changed

• Reworked packages/nvidia_nat_core/src/nat/utils/atif_converter.py into explicit stages:
  • Pass 1: _pass1_build_execution_structure(...)
    • partitions events into root vs delegated child context
    • builds subagent_ref_by_call_id
  • Pass 2: _pass2_project_context_to_steps(...)
    • projects one context’s events into flat ATIF step rows
• Renamed/clarified helper responsibilities:
  • _build_flat_observation_rows(...) (explicit flat observation projection)
  • kept projection flat for nested tools; ancestry remains the nesting signal.
• Kept child trajectories embedded in:
  • trajectory.extra["subagent_trajectories"]
• Simplified ATIFStreamConverter:
  • now a thin wrapper over the same batch two-pass pipeline
  • buffers IST events, reprojects via IntermediateStepToATIFConverter, returns newly visible steps

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• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

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