Keep configured model id for custom providers instead of forcing the default

[scottzx]

Problem

When ~/.codex/config.toml selects a custom provider with a model the built-in catalog doesn't know about (e.g. a self-hosted or third-party endpoint), every turn is sent with the wrong model id and the request fails:

{"error":{"message":"unknown model 'gpt-5.5'","code":"invalid_prompt"}}

…even though the same config works fine through the Codex CLI.

Root cause

CodexAcpClient.createModelId() resolves the configured/returned model id against listModels() and, when it's missing, substitutes availableModels.find(m => m.isDefault):

const selectedModel =
    availableModels.find(m => m.id === modelId) ??
    availableModels.find(m => m.isDefault);   // ← discards the configured model

Custom-provider model ids are not enumerated by listModels(), so the configured model is silently replaced by the built-in default. That default id (response.model → currentModelId) is then pinned onto every runTurn({ model }), so requests to the custom endpoint fail with unknown model '<default>'.

The Codex CLI handles the identical case differently — it keeps the configured model id and only warns:

Model metadata for MiniMax-M3 not found. Defaulting to fallback metadata; this can degrade performance and cause issues.

Fix

Mirror the CLI: when the model id isn't in the advertised catalog, keep it instead of substituting the default. Only fall back to isDefault when no model id was supplied at all. Downstream session state already degrades gracefully for unknown models (supportedReasoningEfforts ?? [], inputModalities ?? ["text", "image"]).

Testing

• Added two unit tests in CodexAcpClient.test.ts for an uncatalogued model id (with an explicit reasoning effort, and defaulting the effort when null).
• vitest run (createModelId suite) and tsc --noEmit pass.
• Verified end-to-end through an ACP client: a session with model = "MiniMax-M3" now advertises MiniMax-M3[high] (previously gpt-5.5) and a prompt turn completes with a real reply from the custom endpoint.