Add comprehensive JSpecify @NullMarked annotations to remaining classes

[Copilot]

Completes JSpecify nullness annotation coverage across the library. Previously ~21 files were annotated; this adds annotations to the remaining ~32 files.

Changes

• Core (Try, DataLoaderOptions, BatchLoaderContextProvider): Mark nullable throwable field, optional builder fields, nullable context return
• Impl: nonNull() accepts @Nullable input; cause() returns @Nullable
• Stats: Context parameters nullable for deprecated method compatibility
• Instrumentation: beginXXX() methods return @Nullable; onCompleted params nullable
• Scheduler: environment param marked @Nullable (null for simple loaders)
• Reactive: callContexts list elements nullable

Type bound fix

// NoOpValueCache now matches ValueCache's type bound
public class NoOpValueCache<K, V extends @Nullable Object> implements ValueCache<K, V>

NullAway suppressions

Two suppressions for known generic type inference limitations:

• AbstractBatchSubscriber.onError() — List<@Nullable Object> constructor
• DataLoaderInstrumentationHelper.whenCompleted() — BiConsumer nullability

Other

• Fixed KotlinExamples.kt explicit type params for stricter nullability
• Updated JSPECIFY_MIGRATION_PLAN.md with completion status

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