fix: loop iterations to forward output from previous iteration

[kausmeows]

Summary

• Fixed a bug where Loop iterations always received the original input instead of the previous iteration's output
• Each iteration's step_input now carries forward from the previous iteration, enabling iterative accumulation patterns
• Fix applied to all 4 execution methods: execute, execute_stream, aexecute, aexecute_stream

fixes: #6862

Type of change

• Bug fix
• New feature
• Breaking change
• Improvement
• Model update
• Other:

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Checklist

• Code complies with style guidelines
• Ran format/validation scripts (./scripts/format.sh and ./scripts/validate.sh)
• Self-review completed
• Documentation updated (comments, docstrings)
• Examples and guides: Relevant cookbook examples have been included or updated (if applicable)
• Tested in clean environment
• Tests added/updated (if applicable)

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Additional Notes

Add any important context (deployment instructions, screenshots, security considerations, etc.)