codex-api: route file uploads through HTTP client factory

[bolinfest]

Why

Codex Apps file parameters use a three-step upload flow: create a file record, PUT bytes to a returned signed URL, and finalize the upload. Each step still constructed a default reqwest client, so the flow could bypass features.respect_system_proxy even after model API requests honored it.

This stack entry makes the resolved client policy a required input to the upload API and resolves each concrete destination independently.

What changed

• Require HttpClientFactory in upload_openai_file.
• Build clients for the create, signed upload, and finalize URLs through the shared API route policy.
• Pass the factory derived from the turn configuration at the Apps/MCP call site.
• Return a destination-aware ClientBuild error when enabled route selection cannot construct a client.
• Preserve the legacy logged fallback for the feature-off ReqwestDefault policy.

Review guide

1. codex-api/src/files.rs changes the upload API and centralizes route-aware client construction.
2. The three request stages each supply their actual URL, including the separately hosted signed blob URL.
3. core/src/mcp_openai_file.rs is the only production caller and supplies the turn configuration factory.

Validation

• cargo check --tests -p codex-api -p codex-core
• just test -p codex-api files (1 matching upload test passed; 135 tests skipped by filter)
• just fix -p codex-api -p codex-core

Follow-up

Other direct HTTP clients remain separate migration slices.

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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