fix: correct spotlessApply path in composite-build violation message

[arimu1]

Problem

Fixes #2421.

When Spotless is applied inside an included build of a Gradle composite, spotlessCheck suggested a bare spotlessApply (historically :spotlessApply from project.path alone). Task name selectors do not match tasks in included builds, so the suggested command does not fix the violations.

For example:

my-composite/
├── settings.gradle   # includeBuild('my-utils')
└── my-utils/
    ├── settings.gradle
    └── build.gradle  # applies Spotless

A violation in my-utils is fixed by ./gradlew :my-utils:spotlessApply, not ./gradlew spotlessApply or ./gradlew :spotlessApply.

Fix

• In the main/root build, keep suggesting bare spotlessApply so one command still fixes every multi-project violation (#2592).
• In an included build (gradle.parent != null), suggest the build-tree task path, e.g. :my-utils:spotlessApply or :my-utils:lib:spotlessApply.
• Prefer Project.getBuildTreePath() (Gradle 8.10+); fall back to reconstructing the path from Gradle.getIncludedBuilds() on older supported versions.

Changes

• plugin-gradle/.../SpotlessCheck.java — composite-aware spotlessApplySelector
• plugin-gradle/.../CompositeBuildRunToFixMessageTest.java — included root, included subproject, and standalone cases
• plugin-gradle/CHANGES.md — Unreleased note linking Logged message to fix formatting violations is incorrect when applied to a composite build #2421

Testing

./gradlew :plugin-gradle:test \
  --tests com.diffplug.gradle.spotless.CompositeBuildRunToFixMessageTest \
  --tests com.diffplug.gradle.spotless.DiffMessageFormatterTest

10/10 passed (JDK 17).