chore: Updating JSpecify coverage

[ehsavoie]

Adding more JSpecify coverage

[gemini-code-assist]

Summary of Changes

Hello @ehsavoie, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the null-safety of the codebase by integrating JSpecify annotations. It involves marking numerous fields, parameters, and return types as potentially nullable, applying package-level null-marking, and configuring Error Prone to enforce these nullability contracts more rigorously. These changes aim to prevent NullPointerExceptions and improve code reliability by making nullability explicit throughout the affected modules.

Highlights

• Extensive Nullability Annotations: Added @Nullable annotations across numerous Java files to fields, method parameters, and return types, explicitly marking where null values are permitted.
• Package-Level Null Marking: Introduced package-info.java files with @NullMarked in several modules, indicating that all types within these packages are considered non-nullable by default unless explicitly annotated with @Nullable.
• Enhanced NullAway Configuration: Updated pom.xml to include new Error Prone checks (-Xep:RequireExplicitNullMarking:WARN and -XepOpt:NullAway:ExhaustiveOverride=true), strengthening compile-time null-safety enforcement.
• Refined Null Handling Logic: Implemented explicit null checks and adjustments in code logic (e.g., returning empty strings instead of null, throwing InternalError on unexpected nulls, handling potentially null taskId in streaming) to align with the new nullability contracts.

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[gemini-code-assist]

Code Review

The pull request primarily focuses on enhancing null-safety across the codebase by introducing JSpecify @Nullable annotations to numerous fields, method parameters, and return types in various classes such as DynamicBomVerifier, RequestContext, EventQueue, TaskManager, and TaskUpdater. New package-info.java files with @NullMarked are added to several packages to enable JSpecify nullness analysis. The pom.xml is updated to include RequireExplicitNullMarking:WARN and ExhaustiveOverride=true for Error Prone. Additionally, the changes include refactoring A2AServerRoutes.java to return an empty string instead of null for getUsername(), adding explicit null checks and error handling in DefaultRequestHandler, ResultAggregator, TaskManager, AsyncExecutorProducer, and AsyncUtils to prevent NullPointerExceptions, and making minor adjustments in the TCK AgentExecutorProducer for task creation and cancellation logic.