GH-3379: Spring Framework 7.x compatibility

[bedrin]

This PR adds compatibility with Spring Framework 7 to enable early adoption

• Use only Spring Framework APIs available both in 6.x and 7.x branches
• Add constructors without logic to *Api classes in models modules to simplify extensibility; effective final fields are marked as final
• Kotlin 2.x support; use kotlin compiler version 2.2.21
• Update MCP SDK to 0.15.0
• Update MCP Annotations to 0.6.0

NOTE: this PR doesn't add support of Spring Boot 4 - it requires a separate and much bigger change and will be carried over in separate PR. This PR doesn't break existing Spring Boot 3.5 / Spring Framework 6 compatibility and can be merged into Spring AI 1.1.x release

[bedrin]

Merged main into this branch - ready for review. This feature brings compatibility with Spring Framework 7.x while keeping it compatible with existing versions (It does NOT support Sprig Boot 4 though). @ilayaperumalg do you think we can merge it to 1.1.x instead? The goal is to allow early adoption and early feedback without having to wait for 2.0 release. In my project I'm not using Spring Boot part of Spring AI so we can already benefit from it.

[filiphr]

Thanks @ericbottard for pointing me to this PR. It is indeed a bit more complete that mine at #4817. However, I believe that some of the things I did in my PR are a bit more performant. e.g. the use of HttpHeaders.readOnlyHttpHeaders is less optimal than the one I am using

additionalHttpHeader.forEach(headers::addAll);

I've left some comments inline.

Additionally, I believe that this PR will still not make the entire codebase compile against Spring 7 and that's because of the HuggingFace generated ApiClient using removed methods in Spring 7. If the ApiClient.mustache from my PR is added then this would be complete.

Additionally, I also added a GitHub action (https://github.com/spring-projects/spring-ai/pull/4817/files#diff-e76e5134b85a9eb3e0d9ce8e671ce97c8c577ff3c24162c45d71691078201347) which compiles the project against Spring 7, so if you want to make sure that Spring AI 1.x stays compatible with Spring 7 I would advise adding that as well.

[filiphr]

This works however it is less performant as it keeps creating new objects even when additionalHttpHeader is empty. I would advise something more like:

additionalHttpHeader.forEach(headers::addAll);

[filiphr]

This can be optimized a bit since at the moment the headers are fetched twice. The getFirst is not needed. The entire section could be

var values = headers.get(headerName);
if (!CollectionUtils.isEmpty(values)) {
    return DurationFormatter.TIME_UNIT.parse(values.get(0));
}

or

var value = headers.getFirst(headerName);
if (value != null) {
    return DurationFormatter.TIME_UNIT.parse(value);
}

[filiphr]

Same comment as for getHeaderAsDuration

[filiphr]

I am not a Kotlin expert, but what does it mean if you use Kotlin compiler 2.2.21? When I tried to compile the project using Kotlin version 1.9.25 it was complaining because Spring Framework 7 had classes that were compiled with 2.2. It could mean that users would have the same problem if they want to use Spring AI 1.x with Spring 6.

[bedrin]

Great catch!

[bedrin]

Thanks @ericbottard for pointing me to this PR. It is indeed a bit more complete that mine at #4817. However, I believe that some of the things I did in my PR are a bit more performant. e.g. the use of HttpHeaders.readOnlyHttpHeaders is less optimal than the one I am using

additionalHttpHeader.forEach(headers::addAll);

I've left some comments inline.

It's already done in PR for Spring Boot 4 support:
https://github.com/spring-projects/spring-ai/pull/4774/files#diff-e5cb77ca54425cc7c57b08f1d70da4118eaf2c0377d381b1d184ace96d6f6e89R417

This PR for Spring Framework 7 support might have some tradeoffs to keep everything compatible with both Spring Framework 6 and 7 but I don't think it makes any noticeable difference.

Additionally, I believe that this PR will still not make the entire codebase compile against Spring 7 and that's because of the HuggingFace generated ApiClient using removed methods in Spring 7. If the ApiClient.mustache from my PR is added then this would be complete.

I did it in separate PR for Spring Boot 4 support but I agree it should be part of Spring 7 support:
https://github.com/spring-projects/spring-ai/pull/4774/files?file-filters%5B%5D=.mustache&show-viewed-files=true#diff-111a294df116430873f6cc4f6fc2e14f67369bb65bd706c65b778cf41ac3ca2c

Additionally, I also added a GitHub action (https://github.com/spring-projects/spring-ai/pull/4817/files#diff-e76e5134b85a9eb3e0d9ce8e671ce97c8c577ff3c24162c45d71691078201347) which compiles the project against Spring 7, so if you want to make sure that Spring AI 1.x stays compatible with Spring 7 I would advise adding that as well.

Need input from maintainers here if it makes sense to have it or keep it simple before 2.0