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