feat(gapic-generator): Add NullMarked annotation to generated classes

[nnicolee]

This PR updates the gapic-generator-java to add the JSpecify @NullMarked annotation to all generated class declarations. Adding @NullMarked at the class level defines the class scope as null-safe, establishing that all unannotated types in method signatures (parameters and return types) are nonnull able by default. This is the first phase in onboarding the generated client libraries to compile-time safety validation, see design doc for more details: go/sdk:java-jspecify-null-annotations-gapic

Classes Annotated:

• Client Classes: AbstractServiceClientClassComposer
• Settings Classes: AbstractServiceSettingsClassComposer and AbstractServiceStubSettingsClassComposer
• Stub Classes: AbstractServiceStubClassComposer and AbstractTransportServiceStubClassComposer
• Callable Factories: AbstractServiceCallableFactoryClassComposer
• Resource Names: ResourceNameHelperClassComposer, CommonStrings

Implementation Changes:

• Added JSpecify import statements to class-level generation templates
• Registered NullMarked.class and Nullable.class in TypeStore across all class composers to ensure references compile and resolve properly

Verification/Testing:

• Please refer to this doc: https://docs.google.com/document/d/1h126pbTGqSwtJyl35E2_vQjM9gNtmWETYCM7PObcvmQ/edit?tab=t.0#heading=h.s4q06jxocx8b

Revisions:

• Mainly to fix the autogenerator, not related to logic: added goldens + dependencies for the build

Next Steps:

• In the next PR, will add @nullable annotations

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
6.2% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud