Add timeout support for ChatClient API

Hi team 👋

I have a suggestion for using ChatClient API.

Expected Behavior

I believe it's going to be great if the ChatClient API support timeout configuration at both global and per-request levels. Here's what I envision:

  // Option 1: Per-request timeout
  ChatResponse response = chatClient
      .prompt("Hello, world!")
      .options(chatOptions)
      .timeout(Duration.ofSeconds(30))
      .call();

  // Option 2: Builder-level default timeout
  ChatClient client = ChatClient.builder(chatModel)
      .defaultTimeout(Duration.ofSeconds(30))
      .build();

  // Option 3: ChatOptions integration
  ChatOptions options = ChatOptions.builder()
      .temperature(0.7)
      .timeout(Duration.ofSeconds(10))
      .build();

If you agree with this feature request, I'll try to contribute it.
Thank you.

Start at the ChatClient API entry point and compare the proposed per-request, builder-level, and ChatOptions timeout scopes. The issue names no files or tests; done means the timeout behavior and precedence are defined and covered across the requested configuration levels.