[Java Harness] Reapply optimization to use Caches.weigh on objects within cached blocks

[scwhittle]

Weighing the lists directly ends up remeasuring all of the objects using measureDeep even if those objects are of a Weighted class.

This was previously done with #36897 but was reverted because it used ImmutableList which doesn't allow null elements and null values are supported otherwise in Beam. Changed to using a array backed list instead and added a unit test for null values.

removed nullness suppression in the file while at it.

Should help #37905

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[gemini-code-assist]

Summary of Changes

Hello, 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 re-enables a critical optimization for cache weighing within the Java Harness, which was previously rolled back. The core improvement involves refactoring the underlying data structures to properly handle null values, a limitation of the prior implementation. This change enhances the efficiency of cache management and improves the robustness of state fetching iterators by ensuring correct weight calculations and null safety.

Highlights

• Reapplied Cache Optimization: Reintroduced an optimization to use Caches.weigh on objects within cached blocks, which was previously reverted due to issues with ImmutableList not supporting null elements.
• Null Value Support: Resolved the ImmutableList issue by switching to an array-backed list for Block values, allowing null elements, and added a dedicated unit test to ensure null values are correctly handled.
• Improved Weight Calculation: Replaced custom arithmetic overflow handling with LongMath::saturatedAdd for weight accumulation in WeightedList and BlocksPrefix, preventing ArithmeticException and ensuring weights cap at Long.MAX_VALUE.
• Nullness Annotations and Suppressions: Cleaned up nullness suppressions and added @Nullable annotations across Caches.java and StateFetchingIterators.java for improved type safety and clarity.
• Refactored Block Class: Converted the StateFetchingIterators.Block from an AutoValue class to a concrete class with a builder pattern, providing more control over its construction and weight calculation.

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