Fix Nullable imports for custom Spring generators

[jorgerod]

Fixes the bug reported in #21070: when using a custom generator whose name does not contain "spring", the Nullable imports were not added.
This change allows extending isSpringCodegen so custom generators can opt into Spring behavior and include the required imports.

Fixes #21070

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Summary by cubic

Fixes missing Nullable imports for custom generators by allowing them to opt into Spring behavior. Changed SpringCodegen.isSpringCodegen() to protected so subclasses can enable Spring-style imports even if their name doesn’t contain “spring” (fixes #21070).

^{Written for commit b5948a6. Summary will update on new commits.}

[cubic-dev-ai]

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