feat: route guardian review model selection through providers

[celia-oai]

Why

Guardian review selection was hard-coded in core, which worked for the default OpenAI path but did not give provider implementations a way to choose backend-specific reviewer model IDs. That matters for Amazon Bedrock: guardian review should run through the Bedrock/Mantle provider using Bedrock's openai.gpt-5.4 model ID, instead of accidentally selecting a reviewer model that implies the OpenAI backend.

What Changed

• Added provider-owned approval review model selection via ModelProvider::approval_review_model_selection.
• Moved the existing default selection policy into the provider abstraction: prefer the requested reviewer model when it is available, otherwise fall back to the active turn model, preferring Low reasoning when supported.
• Added an Amazon Bedrock override that pins guardian review to openai.gpt-5.4 with Low reasoning.

[chatgpt-codex-connector]

💡 Codex Review

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[celia-oai]

@codex review

[chatgpt-codex-connector]

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

ℹ️ About Codex in GitHub

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