Replace Function<X, X> with UnaryOperator<X> in unstable modules

[Copilot]

Replaces all occurrences of Function<X, X> with the more semantically appropriate UnaryOperator<X> in non-stable modules throughout the codebase.

Motivation

UnaryOperator<T> is a specialization of Function<T, T> that more clearly expresses the intent of operations that transform a value of the same type. Using UnaryOperator makes the code more readable and self-documenting.

Changes

This PR updates the following areas:

Core Infrastructure

• TypeTransformer API: Changed Function<Advice.WithCustomMapping, Advice.WithCustomMapping> to UnaryOperator<Advice.WithCustomMapping> in the javaagent extension API and all implementations
• Muzzle: Updated AdviceClassNameCollector to use UnaryOperator

Instrumentation Modules

• Cassandra 4.0/4.4: Changed CompletionStageFunction to implement UnaryOperator<Object>
• Elasticsearch REST 7.0: Updated proxy factory from Function<RestClient, RestClient> to UnaryOperator<RestClient>
• JMX Metrics: Changed unit converter from Function<Number, Number> to UnaryOperator<Number>
• OpenTelemetry API Bridge: Updated context bridging to use UnaryOperator<Object>
• Reactor 3.1: Changed context propagation operators to use:
  • UnaryOperator<Runnable> for schedule hooks
  • UnaryOperator<reactor.util.context.Context> for context transformations
• Resources: Changed MainJarPathFinder to use UnaryOperator<String> for system property lookups

Test Updates

Updated corresponding test files to use UnaryOperator in type declarations and casts where appropriate.

Scope

✅ Only modified unstable modules (no changes to stable APIs)
✅ All affected modules compile successfully
✅ No behavioral changes - purely a refactoring for code clarity

Fixes #[issue-number] (originally suggested in #13776 (comment))

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Explore using UnaryOperator instead of Function<X, X></issue_title>
<issue_description>Originally posted by @trask in #13776 (comment)
</issue_description>

<agent_instructions>Look for all usages of Function<X, X> and replace them with UnaryOperator, as long as the module isn’t marked stable</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #14891

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