Add Spring AI 2.0.x / Spring Boot 4 support

Is your feature request related to a problem? Please describe.
temporal-spring-ai is pinned to Spring AI 1.1.0 / Spring Boot 3.x. Spring AI 2.0 (GA 2026-06-12) requires Spring Boot 4 / Spring Framework 7, and the 1.x→2.x ChatClient/ChatModel/auto-config break is hard — so SB4 + Spring AI 2.0 apps can't use the module at all.

Describe the solution you'd like
A Spring AI 2.0.x / Spring Boot 4 build (2.0 branch or separate artifact), same public surface (TemporalChatClient, ActivityChatModel, the @Tool dispatch types) so usage ports without an API rewrite.

Describe alternatives you've considered
Wasn't aware of this repo at first, so I'm doing my own in-house solution for now. But would be nice to get official modernized temporal + spring-ai sdk support!

https://temporal.io/code-exchange/temporal-spring-ai-integration-framework (doing an approach similar to this example for now, with latest spring-boot + spring-ai)

Additional context
https://github.com/temporalio/sdk-java/tree/master/contrib/temporal-spring-ai

Start by reviewing contrib/temporal-spring-ai and the linked Spring AI integration example, then compare the current Spring AI 1.1.0 / Spring Boot 3.x setup with the 2.0.x / Boot 4 requirements. The work is done when a supported build preserves TemporalChatClient, ActivityChatModel, and the @Tool dispatch types for Spring Boot 4 applications, with validation covering the updated integration.