chore: upgrade all GPT model references to gpt-5.4

GPT no longer has separate codex variants (gpt-5.x-codex), unified as
gpt-5.4. Updated ~26 references across 8 files:

- Codex provider presets: default param + 12 vendor configs
- Codex custom template default model
- Universal provider default Codex model (gpt-4o → gpt-5.4)
- OpenCode @ai-sdk/openai: consolidated 7 old models into single gpt-5.4
- OpenCode RightCode: removed gpt-5.2-codex entry
- OMO agent/category recommended models (7 references)
- Stream check default Codex model
- Universal form fallback and placeholder
- Codex form placeholder text

src/components/providers/forms/CodexFormFields.tsx

@@ -112,7 +112,7 @@ export function CodexFormFields({
             value={modelName}
             onChange={(e) => onModelNameChange(e.target.value)}
             placeholder={t("codexConfig.modelNamePlaceholder", {
-              defaultValue: "例如: gpt-5-codex",
+              defaultValue: "例如: gpt-5.4",
             })}
             className="w-full px-3 py-2 border border-border-default bg-background text-foreground rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 transition-colors"
           />