[SPRING] Improve jspecify Nullable for all args constructor and fluent setters

[jpfinne]

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

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  Commit all changed files.
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Summary by cubic

Improved JSpecify @Nullable handling in Spring generators: we now annotate all-args constructor params and fluent setters, and place annotations correctly inside qualified types and generics. This fixes issues with enums, lists, and Spring Resource, and updates samples/tests; Spring Boot 4 output now uses java.time.Instant where configured.

• New Features

  • When useJspecify=true, generate @Nullable on all-args constructor parameters and fluent chain setters in Spring POJOs.
  • Added nullableArgumentWithEnum.mustache to support enum datatypes in constructor/setter arguments.

• Bug Fixes

  • Correct placement of @Nullable within qualified types and generics by improving jSpecifyDatatype (e.g., java.time.@Nullable Instant, @Nullable List<java.time.Instant>).
  • Updated spring-boot-4-jspecify.yaml type mappings: OffsetDateTime -> java.time.Instant, BigDecimal -> java.math.BigDecimal; expanded tests and regenerated samples.

^{Written for commit 9fdd676. Summary will update on new commits.}