Honor Codex proxy-owned model selection

[kywch]

Summary

• treat exact CODEX_CONFIG.model ownership as authoritative
• skip ACP set_model only when proxy config proves it owns requested model
• fail closed to normal set_model behavior for missing, empty, malformed, non-object, missing-model, or mismatched config

Validation

• 109 focused tests passed
• Ruff, format, and git diff --check passed
• exact-head arXiv integration condition codex-gpt54 completed at reward 1 with no provider, sandbox, or trial errors
• full 10-condition matrix completed; sole zero reward was unrelated Gemini provider 429

Review

Maintainer or other-person review required. Author will not self-merge.

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[devin-ai-integration]

Devin Review found 1 potential issue.

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[kywch]

Addressed the proxy-owned Codex effort finding. Requested model_reasoning_effort now enters launch config without unsupported ACP model/effort calls. Regression covers no-effort and high; all CI checks pass.