feat(gapic-generator): add JSpecify Null annotations to the generator classes

[nnicolee]

This PR continues the onboarding of the gapic-generator-java project to JSpecify null-safety. Specifically, it applies the @NullMarked annotation to all modified source and test classes in this branch to declare them non-null by default.

Annotations Applied

• @NullMarked Class Declarations: Applied at the class level to all modified source and test classes to establish a non-null by default context.
• @nullable Type Annotations: Applied to specific methods, fields, and parameters to explicitly denote where null values are accepted/returned.

Automation Method
To apply the class-level annotations efficiently, we leveraged Chris's Error Prone automation tool (custom BugChecker plugin under com.google.errorprone.bugpatterns.nullness) to run AST-based refactoring and automatically insert the JSpecify @NullMarked annotations across the codebase.

Verification/Testing

• Please refer to this doc: (link tba)

[sonarqubecloud]

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

Failed conditions
B Maintainability Rating on New Code (required ≥ A)
C Reliability Rating on New Code (required ≥ A)

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[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