fix(core): consume iterate inputs once

[Eilen6316]

Fix repeated external inputs across iterative execution APIs.

Changes

• Consume inputs only on the first step of iterate and aiterate.
• Apply the same behavior to stream_iterate and astream_iterate.
• Preserve inputs for first-step halt-before, retries, and resumed failed positions.
• Update human-in-the-loop documentation and regression coverage.

How I tested this

• pytest -q tests/core/test_application.py (147 passed)
• black, isort, flake8, and Python AST checks passed on changed files.

Notes

This changes behavior for callers that relied on external inputs being forwarded to later actions in the same run.