@langchain/openai should let user define isReasoningModel

[nightire]

@langchain/openai decides which model supports reasoning by using this function currently:

langchainjs/libs/langchain-openai/src/chat_models.ts

Lines 124 to 129 in 9eb78b7

	function isReasoningModel(model?: string) {
	if (!model) return false;
	if (/^o\d/.test(model ?? "")) return true;
	if (model.startsWith("gpt-5") && !model.startsWith("gpt-5-chat")) return true;
	return false;
	}

This is fine if using the official models. However, with the 3rd-party providers, such as openrouter, this function prevents users to use models with reasoning support.

This is an example:

const openai = new ChatOpenAI({
  configuration: {
    apiKey: import.meta.env.VITE_OPENAI_API_KEY,
    baseURL: import.meta.env.VITE_OPENAI_BASE_URL,
  },
  model: "openai/gpt-5",
  frequencyPenalty: 0.1,
  presencePenalty: 0.1,
  maxTokens: 2000,
  // reasoning: { effort: "minimal", summary: "concise" },
  temperature: 0,
  topP: 0.1,
  useResponsesApi: true,
  verbose: false,
  verbosity: "low",
})

This works fine only except the reasoning because the model name openai/gpt-5 could never pass the check function.

I suggest to support passing a custom check function or regex pattern to determine which is a reasoning model. Is this a valid point?