chore: Migrate to Spring Boot 4.0.1 and Spring AI 2.0.0-M1

[pacphi]

Summary

This PR migrates the project to Spring Boot 4.0.1 and Spring AI 2.0.0-M1, addressing breaking changes in Spring Boot 4.x APIs.

Changes

Build Configuration Updates

• Spring Boot: 4.0.0 → 4.0.1
• Spring AI: 1.0.0 → 2.0.0-M1
• Added Spring milestone repository for Spring AI 2.0.0-M1 artifacts
• Changed Java toolchain configuration to sourceCompatibility/targetCompatibility

Code Migration Changes

Http.java - RestClient Configuration

Simplified RestClient.Builder configuration for Spring Boot 4.x compatibility:

// Updated from RestClientCustomizer to direct RestClient.Builder bean
@Bean
public RestClient.Builder restClientBuilder() {
    SimpleClientHttpRequestFactory factory = new SimpleClientHttpRequestFactory();
    factory.setConnectTimeout(Duration.ofMinutes(10));
    factory.setReadTimeout(Duration.ofMinutes(10));
    
    return RestClient.builder()
            .requestFactory(factory)
            .defaultHeader(HttpHeaders.ACCEPT_ENCODING, "gzip, deflate");
}

MultiChat.java - RetryTemplate Import

Changed RetryTemplate import path (Spring AI 2.0 relocation):

// Before
import org.springframework.retry.support.RetryTemplate;

// After  
import org.springframework.core.retry.RetryTemplate;

Settings Updates

• Removed snapshot repository from settings.gradle (using milestone only for Spring AI 2.0.0-M1)

Validation

• ✅ Build: PASSED (./gradlew clean build)
• ✅ All compilation issues resolved
• ✅ No test failures (no tests in source)

Test Plan

• Run full build verification
• Test OpenRouter API integration
• Verify multi-model chat functionality
• Test HTTP client timeout configuration
• Validate actuator endpoints

Files Modified

• build.gradle - Version updates
• settings.gradle - Repository configuration
• src/main/java/me/pacphi/config/Http.java - RestClient configuration
• src/main/java/me/pacphi/config/MultiChat.java - RetryTemplate import

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