null-safety

# Null Safety with JSpecify (Boot 4) Spring Framework 7 adopts JSpecify for nullability. Use `org.jspecify.annotations` for new code and migrate away from the older Spring nullability annotations at module boundaries. ## Mark packages and annotate exceptions ```java @NullMarked package com.example.orders; import org.jspecify.annotations.NullMarked; ``` Inside a null-marked package, types are non-null by default. Annotate only genuine exceptions: ```java @Nullable Order findActive(String customerEmail) { ... } List<@Nullable String> namesWithGaps; // non-null list, nullable elements @Nullable List<String> maybeNoList; // nullable list, non-null elements ``` Use `Object @Nullable []` for a nullable array and `@Nullable Object[]` for a non-null array whose elements may be null. Add the JSpecify dependency through the Boot dependency management. JSpecify is metadata. IntelliJ can inspect it, while NullAway with `JSpecifyMode=true` can enforce the contract during compilation. Kotlin consumes the annotations as real nullability. ## Gotchas - Agent uses `org.springframework.lang.Nullable` in new Framework 7 code - prefer JSpecify. - Agent annotates every parameter with `@NonNull` - use package-level `@NullMarked` and mark exceptions. - Agent writes `@Nullable List<String>` for nullable elements - use `List<@Nullable String>`. - Agent puts `@Nullable` on the wrong side of an array type - distinguish nullable arrays from nullable elements. - Agent expects annotations alon...