model-provider: route model discovery through HTTP client factory

[bolinfest]

Why

Model discovery is a startup-critical request path, but it still constructed a default reqwest client directly. That meant features.respect_system_proxy could apply to Responses API traffic while /models catalog refreshes bypassed the same resolved proxy policy.

This is the next focused migration on top of #31342. It makes the proxy decision mandatory at the model-provider boundary rather than adding another optional flag a caller could omit.

What changed

• Require an HttpClientFactory when constructing a ModelProvider models manager.
• Pass the factory resolved from Config through codex-core into the OpenAI models endpoint.
• Build the /models transport with the route-aware API client and the full request destination.
• Keep the Amazon Bedrock provider unchanged functionally because its model catalog is static.
• Give test-only construction an explicit ReqwestDefault policy.

Review guide

1. model-provider/src/provider.rs adds the required factory to the provider API.
2. model-provider/src/models_endpoint.rs is the behavior change that routes /models through the shared client policy.
3. core/src/thread_manager.rs supplies the feature-derived factory; the remaining edits update test and alternate-provider construction.

Validation

• cargo check --tests -p codex-model-provider -p codex-core -p codex-memories-write
• just test -p codex-model-provider (50 tests passed)

Follow-up

Other direct reqwest consumers will be migrated in separate, reviewable stack entries.

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Stack created with Sapling. Best reviewed with ReviewStack.

• codex-api: route file uploads through HTTP client factory #31363
• core: route realtime and memories through HTTP client factory #31362
• -> model-provider: route model discovery through HTTP client factory #31361
• core: use HTTP Responses with system proxy #31342