Support `labels` for `ChatGoogleGenerativeAI`

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Issue Content

ChatGoogleGenerativeAI should support labels parameter for cost tracking and traceability

Example Code

import { ChatGoogleGenerativeAI } from "@langchain/google-genai";

const model = new ChatGoogleGenerativeAI({
  model: "gemini-2.0-flash",
  labels: { env: "development" },
});

Expected Behavior

The labels parameter should be accepted and passed through to sdk to tag LLM calls with metadata for cost aggregation and traceability purposes.

ChatVertexAI in @langchain/google-vertexai already supports a labels parameter for this purpose (added for cost aggregation and billing traceability).

For parity and to support users who need to track costs across different environments or applications, ChatGoogleGenerativeAI should also accept and forward the labels parameter to the underlying SDK.

Related

• Python equivalent issue/PR: feat(genai): add labels support langchain-google#1503
• ChatVertexAI labels implementation in @langchain/google-vertexai