feat(gapic-generator): Add NullMarked annotation to generated classes (#13517)

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](http://goto.google.com/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

Author: nnicolee

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/ComplianceClient.java

@@ -41,6 +41,7 @@
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -355,6 +356,7 @@
  *
  * <p>Please refer to the GitHub repository's samples for more quickstart code snippets.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class ComplianceClient implements BackgroundResource {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/ComplianceSettings.java

@@ -43,6 +43,7 @@
 import java.io.IOException;
 import java.util.List;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -93,6 +94,7 @@
  * Guide](https://docs.cloud.google.com/java/docs/client-retries) for additional support in setting
  * retries.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class ComplianceSettings extends ClientSettings<ComplianceSettings> {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/EchoClient.java

@@ -49,6 +49,7 @@
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -364,6 +365,7 @@
  *
  * <p>Please refer to the GitHub repository's samples for more quickstart code snippets.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class EchoClient implements BackgroundResource {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/EchoSettings.java

@@ -49,6 +49,7 @@
 import java.io.IOException;
 import java.util.List;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -124,6 +125,7 @@
  *     .build();
  * }</pre>
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class EchoSettings extends ClientSettings<EchoSettings> {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/IdentityClient.java

@@ -42,6 +42,7 @@
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -286,6 +287,7 @@
  *
  * <p>Please refer to the GitHub repository's samples for more quickstart code snippets.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class IdentityClient implements BackgroundResource {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/IdentitySettings.java

@@ -45,6 +45,7 @@
 import java.io.IOException;
 import java.util.List;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -95,6 +96,7 @@
  * Guide](https://docs.cloud.google.com/java/docs/client-retries) for additional support in setting
  * retries.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class IdentitySettings extends ClientSettings<IdentitySettings> {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/MessagingClient.java

@@ -50,6 +50,7 @@
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -449,6 +450,7 @@
  *
  * <p>Please refer to the GitHub repository's samples for more quickstart code snippets.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class MessagingClient implements BackgroundResource {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/MessagingSettings.java

@@ -50,6 +50,7 @@
 import java.io.IOException;
 import java.util.List;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -125,6 +126,7 @@
  *     .build();
  * }</pre>
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class MessagingSettings extends ClientSettings<MessagingSettings> {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/SequenceServiceClient.java

@@ -43,6 +43,7 @@
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -309,6 +310,7 @@
  *
  * <p>Please refer to the GitHub repository's samples for more quickstart code snippets.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class SequenceServiceClient implements BackgroundResource {

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/SequenceServiceSettings.java

@@ -45,6 +45,7 @@
 import java.io.IOException;
 import java.util.List;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
 
 // AUTO-GENERATED DOCUMENTATION AND CLASS.
 /**
@@ -96,6 +97,7 @@
  * Guide](https://docs.cloud.google.com/java/docs/client-retries) for additional support in setting
  * retries.
  */
+@NullMarked
 @BetaApi
 @Generated("by gapic-generator-java")
 public class SequenceServiceSettings extends ClientSettings<SequenceServiceSettings> {