fix(models): use 1M context for openai-codex gpt-5.4 (#37876)

Merged via squash.

Prepared head SHA: c41020779e
Co-authored-by: yuweuii <82372187+yuweuii@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/agents/pi-embedded-runner/model.test-harness.ts

@@ -36,13 +36,14 @@ export function mockOpenAICodexTemplateModel(): void {
 export function buildOpenAICodexForwardCompatExpectation(
   id: string = "gpt-5.3-codex",
 ): Partial<typeof OPENAI_CODEX_TEMPLATE_MODEL> & { provider: string; id: string } {
+  const isGpt54 = id === "gpt-5.4";
   return {
     provider: "openai-codex",
     id,
     api: "openai-codex-responses",
     baseUrl: "https://chatgpt.com/backend-api",
     reasoning: true,
-    contextWindow: 272000,
+    contextWindow: isGpt54 ? 1_050_000 : 272000,
     maxTokens: 128000,
   };
 }