[Spring-http-interface] JSpecify: fix api argument with default value

[jpfinne]

fix #24737

For spring-http-interface only: the api arguments with default value are annotated with @Nullable.
Spring will inject the specified default value if null is provided.

Small refactoring of the spring templates:
nullableAnnotation.mustache is kept for model only
nullableAnnotation_apiArgument.mustache is introduced for all api arguments (queryParams, pathParams, headerParams)

This make it easier to redefine nullableAnnotation_apiArgument for spring-http-interface to handle the default parameters.

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Summary by cubic

Marks spring-http-interface API parameters with a default value as @Nullable and fixes JSpecify handling to avoid duplicate @Nullable. Previously, defaulted @RequestParam/@RequestHeader were not nullable; now they are for spring-http-interface only. No runtime change—Spring still injects the default when null is provided.

• Introduces nullableAnnotation_apiArgument.mustache plus a spring-http-interface override, and switches body, query, header, cookie, and path templates to use it.
• Simplifies nullableAnnotation.mustache to pick JSpecify cleanly and updates nullableAnnotation_jspecify.mustache to prevent double @Nullable and handle default vs non-default cases.
• Updates tests to assert the change is scoped to spring-http-interface and regenerates samples.

Migration

• If you enable JSpecify, ensure org.jspecify:jspecify is on the classpath.

^{Written for commit 4e821b1. Summary will update on new commits.}

[anthonyvdotbe]

@jpfinne is this ready for review? If not, what remains to be done? I'd like to get this fixed ASAP, so if needed I can create a PR myself

[jpfinne]

@jpfinne is this ready for review? If not, what remains to be done? I'd like to get this fixed ASAP, so if needed I can create a PR myself

@anthonyvdotbe I've simplified the nullable annotations to easily distinguish between model and api.
Can you validate and approve the PR?

We are close to the release date... Contact wing328 to accelerate the merge

[cubic-dev-ai]

All reported issues were addressed across 17 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[anthonyvdotbe]

@anthonyvdotbe I've simplified the nullable annotations to easily distinguish between model and api. Can you validate and approve the PR?

@jpfinne I see the AI bot suggested changes, so I'll let you resolve those first

[jpfinne]

@anthonyvdotbe I've simplified the nullable annotations to easily distinguish between model and api. Can you validate and approve the PR?

@jpfinne I see the AI bot suggested changes, so I'll let you resolve those first

@anthonyvdotbe done

[anthonyvdotbe]

In general, I'd prefer a more focused PR that just fixes this particular issue and leaves the refactoring for another PR.