cleanup(gax-java): remove javax.annotation.Nonnull usage

[nnicolee]

This PR removes all occurrences of legacy javax.annotation.Nonnull annotations and imports across the gax-java codebase, removes the unused jsr305 dependency, and corrects misplaced/incorrect nullability annotations in the core modules (gax, gax-grpc, and gax-httpjson).

Why this change is needed:

As part of the repository-wide migration to JSpecify annotations, packages/classes are annotated with @NullMarked, making all unannotated types non-nullable by default.

• The legacy javax.annotation.Nonnull annotations are redundant under JSpecify semantics and can be safely removed.
• Resolving misplaced nullability signatures and aligning overridden methods enables compilation checks under NullAway static analysis with JSpecifyMode=true.

JSR-305 (jsr305) Dependency Removal:

We removed the com.google.code.findbugs:jsr305 dependency from the pom.xml files. This is not a breaking change for the following reasons:

• Why it was removed: Because we removed all references to the javax.annotation package in the codebase, the Maven dependency analyzer (mvn dependency:analyze) flagged jsr305 as an unused declared dependency. Since our verification fails on any warning (-DfailOnWarning=true), the dependency had to be removed to pass presubmit checks.
• Why it's not a breaking change: The dependency was declared with provided scope, meaning it was only used during compilation and was never distributed transitively to downstream users. Removing it has no runtime impact on library consumers.

Changes:

• Removed import javax.annotation.Nonnull; and @Nonnull from 30 files across gax, gax-grpc, and gax-httpjson (including test files).
• Migrated remaining javax.annotation.Nullable usages in tests to JSpecify's @Nullable type-use annotation.
• Removed the unused jsr305 dependency from the gax-java module POMs.
• Fixed type-use annotation placements and annotate missing annotations
• Formatted all modified files using fmt-maven-plugin.
• Verified that all compilation and dependency analysis stages pass cleanly.

[gemini-code-assist]

Code Review

This pull request removes the @Nonnull annotation and its corresponding javax.annotation.Nonnull import across numerous files in the gax-java library, likely as part of a migration to JSpecify annotations. A review of the changes identified a redundant null check in ServerStreamingCallSettings.java where Preconditions.checkNotNull(resumptionStrategy) is called twice consecutively, which should be simplified.

[lqiu96]

@nnicolee I think there are a few straggling instances I've seen with @Nullable being in the wrong spot from doing a spot check:

google-cloud-java/sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

Line 269 in 9bf1eb5

@Nullable

Can you take a look? I think we can also raise a new PR to do additional cleanup

[lqiu96]

/gemini review

[gemini-code-assist]

Code Review

This pull request migrates the codebase from JSR-305 nullness annotations (@nonnull, @nullable) to JSpecify annotations (@nullable, @NullMarked). It removes the jsr305 dependency from the Maven POM files across multiple modules (gax-grpc, gax-httpjson, and gax) and updates the Java source files by removing @nonnull annotations (relying on class-level @NullMarked instead) and shifting @nullable annotations to type-use positions. There are no review comments to address, and I have no additional feedback to provide.

[sonarqubecloud]

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

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

See analysis details on SonarQube Cloud

Catch issues before they fail your Quality Gate with our IDE extension SonarQube for IDE

[sonarqubecloud]

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

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
B Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

Catch issues before they fail your Quality Gate with our IDE extension SonarQube for IDE