Honor effective Gradle Isolated Projects flags in KSP source wiring

[gregjotau]

fix #3188

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[gregjotau]

The current property-only check does not see --isolated-projects, and it ignores --no-isolated-projects overriding a property. This patch uses injected BuildFeatures.isolatedProjects.active on Gradle 8.5+ and keeps the existing property fallback on older Gradle versions. The explicit ksp.project.isolation.enabled=true opt-in is preserved even when Gradle isolation is disabled.

Before the implementation change, the focused tests reproduced both wrong values: the CLI enable test returned false and the CLI disable-over-property test returned true. With the change, all six tests pass, with no skips:

• CLI enable and configuration-cache reuse
• CLI disable overriding the Gradle property
• explicit KSP opt-in with isolation disabled
• Gradle 8.4 compatibility before BuildFeatures exists
• the legacy isolation property on Gradle 8.5
• a two-project processor/consumer build compiling actual generated Kotlin classes with only --isolated-projects, then reusing its configuration cache

Validation (JDK 17):

./gradlew :gradle-plugin:validatePlugins :gradle-plugin:test --tests '*ProjectIsolationTest*'

The changed Kotlin files also pass ktfmt 0.63 with the repository's kotlinlang style. Tests consume the build's test Maven publications, using the real plugin classpath and no TestKit classloader injection. No repository-wide test claim is made.

This patch and its tests were substantially prepared and tested with Codex. The draft is available for human review; any required contributor agreement must be completed by the contributor, not the agent.

[gregjotau]

@Mon322812 Can you just open this and get it merged, I dont care about contribution here and the CLA. I just want it fixed :)