Generate required fields constructor when JSpecify is enabled

[sar9ho]

Adds a “required fields” constructor for Java types when generateJSpecifyAnnotations = true.

Behavior:

• If all fields are nullable, generate a public no-arg constructor (Default constructor could be visible when JSpecify is enabled with all nullable fields #900)
• If any fields are non-nullable, generate a private no-arg constructor (builder-only) and a public constructor with only the non-nullable fields (Consider generating a non-nullable fields constructor when JSpecify is enabled #901)
• Keep existing all-args constructor behavior (and its nullability annotations) so existing usage/tests don’t change

For testing, I added some targeted cases in CodeGenTest ^

Fixes #900
Fixes #901

[sjohnr]

Thanks for the PR @sar9ho! This looks like a good start, but I have some suggestions for changes below.

[sjohnr]

This change would be better included under the existing javaGenerateAllConstructor check, so that generating one (or more) multi-arg constructors is still tied to that configuration item. It's not actually necessary to link the required-args constructor to JSpecify (generateJSpecifyAnnotations).

Alternatively, I am wondering if it might be better to add a new config item (javaGenerateRequiredFieldsConstructor) to control adding this new constructor instead of automatically adding it.

In either case, there should also be a check for the required-fields constructor to ensure it's not added if all fields are required (fields.size == requiredFields.size). It would be good to include a test for this case as well.