Prompt to enable JSpecify null analysis

[ziplla]

Fixes #1715

This change implements the focused fix and regression coverage described in the issue.

Validation: the repository pull-request workflow passed: https://github.com/ziplla/spring-tools/actions/runs/30659169388

[BoykoAlex]

I have my doubts about this.

1. This is probably for JDT UI not for spring tools
2. I'd try to look for an alternative to ui.startup extension to activate this. Likely moving this to JDT UI would resolve my concern implicitly

[ziplla]

The prompt is JDT-wide rather than Spring-specific, and removing the startup extension meaningfully depends on where the feature should live. Please confirm whether this PR should be closed in favor of an upstream JDT UI contribution or retained here using a Spring Tools activation hook.

[martinlippert]

I think we should at least see if the folks working on JDT UI would be interested in this. I think I haven't seen this kind of feature/compiler activation prompts in JDT UI yet, so this might raise questions and discussions how this kind of feature should be implemented, but that is probably a good thing which could drive consistency in Eclipse.

@ziplla Would you go ahead and raise that question / PR with JDT UI and link to it from here?