Add support for multiple named OpenAI chat models in Spring Boot starter

[ghtjr410]

Issue

Closes langchain4j/langchain4j#4286

Change

Add support for multiple named OpenAI chat models in Spring Boot starter.

Configuration

# Global settings (inherited by all named models)
langchain4j.open-ai.chat-model.api-key=${OPENAI_API_KEY}

# Named models
langchain4j.open-ai.chat-model.fast.model-name=gpt-4.1-mini
langchain4j.open-ai.chat-model.smart.model-name=gpt-5.2

Usage

@AiService(wiringMode = EXPLICIT, chatModel = "openAiChatModelFast")
interface FastAssistant { }

@AiService(wiringMode = EXPLICIT, chatModel = "openAiChatModelSmart")
interface SmartAssistant { }

Implementation

• Uses BeanDefinitionRegistryPostProcessor to dynamically register named model beans
• Scans Environment for unknown property keys under chat-model.* prefix
• Supports global property inheritance (api-key, base-url, etc.)
• No changes to Properties.java - 100% backwards compatible
• Named models support ChatModelListener callbacks and Spring RestClient integration

Bean Naming Convention

Configuration	Bean Name
chat-model.fast.*	openAiChatModelFast
chat-model.4-1-mini.*	openAiChatModel41Mini

General checklist

• There are no breaking changes
• I have added unit and/or integration tests for my change
• The tests cover both positive and negative cases
• I have manually run all the unit and integration tests in the module I have added/changed, and they are all green
• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)

Checklist for adding new Spring Boot starter

N/A - This PR modifies an existing starter.

[dliubarskyi]

@ThomasVitale I would appreciate a review from you, if you have some spare time 🙏