Add Streams.forEachWithIndex

[sri-adarsh-kumar]

Summary

• add Streams.forEachWithIndex(Stream, ObjLongConsumer) as a terminal counterpart to mapWithIndex
• delegate through mapWithIndex so indexed traversal stays on one code path in both JRE and Android Streams
• add StreamsTest coverage for sized, unsized, null-element, null-check, and parallel index-correctness cases

Context

• addresses Streams.forEachWithIndex(*Stream, *ConsumerWithIndex)? #2913
• this follows the issue discussion by using JDK ObjLongConsumer rather than introducing a new ConsumerWithIndex

Intentional Non-Goals

• no primitive overloads in this PR
• no stronger parallel ordering guarantee than correct element-to-index correspondence
• no implicit closing of the source stream

Test Plan

• Ran ./mvnw -pl guava-tests -Dtest=StreamsTest -DfailIfNoTests=false -Dmaven.javadoc.skip test

GenAI Usage

• I used GenAI assistance while drafting parts of the implementation and tests.
• I manually reviewed the final API shape, semantics, and test coverage before opening the PR.

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