feat: add openai model info configuration (#551)

In reference to [Issue 548](https://github.com/openai/codex/issues/548)
- part 1.

codex-cli/src/utils/model-utils.ts

@@ -2,6 +2,7 @@ import type { ResponseItem } from "openai/resources/responses/responses.mjs";
 
 import { approximateTokensUsed } from "./approximate-tokens-used.js";
 import { getBaseUrl, getApiKey } from "./config";
+import { type SupportedModelId, openAiModelInfo } from "./model-info.js";
 import OpenAI from "openai";
 
 const MODEL_LIST_TIMEOUT_MS = 2_000; // 2 seconds
@@ -89,10 +90,12 @@ export async function isModelSupportedForResponses(
 }
 
 /** Returns the maximum context length (in tokens) for a given model. */
-function maxTokensForModel(model: string): number {
-  // TODO: These numbers are best‑effort guesses and provide a basis for UI percentages. They
-  // should be provider & model specific instead of being wild guesses.
+export function maxTokensForModel(model: string): number {
+  if (model in openAiModelInfo) {
+    return openAiModelInfo[model as SupportedModelId].maxContextLength;
+  }
 
+  // fallback to heuristics for models not in the registry
   const lower = model.toLowerCase();
   if (lower.includes("32k")) {
     return 32000;