Add timeout support for ChatClient API

[HyunSangHan]

Implemented timeout support for the ChatClient API.

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

// Option 2: ChatOptions integration (medium priority)
ChatOptions options = ChatOptions.builder()
    .temperature(0.7)
    .timeout(Duration.ofSeconds(10))
    .build();

// Option 3: Builder-level default timeout (lowest priority)
ChatClient client = ChatClient.builder(chatModel)
    .defaultTimeout(Duration.ofSeconds(30))
    .build();

The implementation introduces three levels of timeout configuration with proper priority handling:

1. Request-level timeout
   Added timeout(Duration) method to ChatClientRequestSpec for per-request configuration (highest priority).

2. ChatOptions-level timeout
   Added getTimeout() accessor and timeout(Duration) builder method to the ChatOptions interface (medium priority).

3. Builder-level default timeout
   Added defaultTimeout(Duration) method to the ChatClient.Builder for global default configuration (lowest priority).

I confirmed relevant tests are passed like below:

Closes: #4103