[SUREFIRE-823] Decouple -DskipTests from Failsafe plugin

[olamy]

The Failsafe plugin's skipTests parameter is no longer bound to the
skipTests user property. -DskipTests now only skips Surefire (unit tests)
while leaving Failsafe (integration tests) untouched.

Use -DskipITs to skip integration tests from the command line, or
-Dmaven.test.skip to skip both. The plugin configuration
element is still honoured when set explicitly.

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[Copilot]

Pull request overview

This PR implements SUREFIRE-823 by changing the Maven Failsafe Plugin so that the -DskipTests user property no longer skips integration tests; it now only affects the Maven Surefire Plugin (unit tests). The change is validated via a new integration test fixture and accompanying IT, and is documented in both plugin sources and site docs.

Changes:

• Decouple Failsafe’s legacy skipTests parameter from the skipTests user property by removing @Parameter(property = "skipTests") bindings and keeping <skipTests> config support via a deprecated, config-only parameter.
• Add a new integration-test fixture project and IT coverage to verify behavior for -DskipTests, -DskipITs, and -Dmaven.test.skip.
• Update user-facing documentation and Javadocs to describe the new skip semantics.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
surefire-its/src/test/resources/failsafe-1766-skip-decoupled/src/test/java/MyTest.java	Adds a unit test class used by the new IT fixture to validate Surefire behavior.
surefire-its/src/test/resources/failsafe-1766-skip-decoupled/src/test/java/MyIT.java	Adds an integration test class used by the new IT fixture to validate Failsafe behavior.
surefire-its/src/test/resources/failsafe-1766-skip-decoupled/pom.xml	Defines a minimal project using both Surefire and Failsafe to exercise skip behavior end-to-end.
surefire-its/src/test/java/org/apache/maven/surefire/its/jiras/Surefire1766SkipTestsDecoupledIT.java	Adds integration tests asserting skipTests only skips Surefire, while skipITs / maven.test.skip behave as expected.
maven-surefire-plugin/src/site/apt/examples/skipping-tests.apt.vm	Updates documentation to explain the new -DskipTests vs -DskipITs semantics (one section still needs correction per review comment).
maven-surefire-common/src/main/java/org/apache/maven/plugin/surefire/AbstractSurefireMojo.java	Updates Javadoc on skipTests to reflect that Failsafe no longer binds to the skipTests user property.
maven-failsafe-plugin/src/main/java/org/apache/maven/plugin/failsafe/VerifyMojo.java	Makes skipTests config-only (not bound to the user property) and deprecates it in favor of skipITs.
maven-failsafe-plugin/src/main/java/org/apache/maven/plugin/failsafe/IntegrationTestMojo.java	Introduces a deprecated, config-only skipTests parameter and overrides isSkipTests() so -DskipTests no longer skips IT execution.