Fix IllegalMutationException when root project uses predeclareDeps() with format tasks

Fixes

• Gradle Plugin: predeclareDeps DefaultTaskContainer#withType(Class, Action) on task set cannot be executed in the current context #2885

Introduced by

• Partially support isolated projects #2854

Problem

PR #2854 ("Partially support isolated projects") refactored SpotlessTaskService.hookSubprojectTask to look up the RegisterDependenciesTask dynamically via the task container rather than holding a direct reference. The call used withType(Class, Action):

project.getRootProject().getTasks()
  .withType(RegisterDependenciesTask.class, task -> task.hookSubprojectTask(task));

On Gradle 8.14.x (unsure about the exact version this behavior was introduced), this throws IllegalMutationException when all the following are true:

1. The root project calls predeclareDeps()
2. The root project also configures its own spotless format tasks (e.g. java, groovyGradle)
3. A spotless task on the root project is being realized

During root project task realization, Gradle activates its mutation guard on the root task container.
The configure callback calls FormatExtension.setupTask() then hookSubprojectTask() then project.getRootProject().getTasks().withType(Class, Action) on that same guarded container which Gradle forbids.

Switching to withType(Class).configureEach(Action) does not help either as Gradle also considers configureEach a mutating registration in this context.

Fix

Since RegisterDependenciesTask is the task that must run before all projects, this fix assumes it can be safely referenced by the main spotless service. So the fix stores the RegisterDependenciesTask reference directly in SpotlessTaskService at configuration time (when SpotlessExtensionPredeclare is constructed and isUsingPredeclared() is set). Then hookSubprojectTask uses that reference directly, with no task container lookup during task realization.

// SpotlessExtensionPredeclare constructor
taskService.registerDependenciesTask = this.registerDependenciesTask;

// hookSubprojectTask — no task container query
if (registerDependenciesTask != null) {
    registerDependenciesTask.hookSubprojectTask(task);
}

The RegisterDependenciesTask reference is already computed eagerly in SpotlessExtensionPredeclare (via findRegisterDepsTask().get()), so no new eagerness is introduced.

Note that IsolatedProjectTest.predeclaredIsUnsupported() (introduced in #2854) expected a build failure that no longer occurs because this fix`

Verification

Added MultiProjectTest.predeclaredDepsRegression() that reproduces the exact trigger conditions and runs against Gradle 8.14.