fix: store the iteration index consistently

[njzjz-bot]

Summary

• Make the index_iteration setter initialize the same backing attribute read by its getter.
• Remove the no-longer-needed missing-attribute suppression.
• Add a construction and round-trip regression test.

Tests

• python -m unittest tests.tools.test_auto_gen_param -v
• ruff check dpgen/tools/auto_gen_param.py tests/tools/test_auto_gen_param.py
• ruff format --check dpgen/tools/auto_gen_param.py tests/tools/test_auto_gen_param.py

Fixes #1893

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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Configuration used: Organization UI

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Run ID: 9f99fc03-f602-4fe4-8fe4-f0c01739af9e

📥 Commits

Reviewing files that changed from the base of the PR and between d5ce577 and 9a4e923.

📒 Files selected for processing (2)

• dpgen/tools/auto_gen_param.py
• tests/tools/test_auto_gen_param.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.17%. Comparing base (d5ce577) to head (9a4e923).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
+ Coverage   49.80%   50.17%   +0.36%     
==========================================
  Files          83       83              
  Lines       14986    14986              
==========================================
+ Hits         7464     7519      +55     
+ Misses       7522     7467      -55     

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