fix: omit chat completions store by default

[Aphroq]

Summary

Fixes Chat Completions store default handling so ModelSettings(store=None) no longer sends store=True for the official OpenAI client.

ModelSettings.store documents that Chat Completions is disabled when not specified, but ChatCmplHelpers.get_store_param() previously opted official OpenAI clients into store=True. This change returns the explicit setting as-is, preserving explicit store=True and store=False while omitting the parameter when unset.

Test plan

• uv run pytest tests/models/test_openai_chatcompletions.py -k 'store_param or fetch_response'
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3103

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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[seratch]

Closing this due to #3103 (comment)