Regression in 8.6.0: groovyGradle/greclipse fails with "roundtripStateInternal or equalityStateInternal should be non-null" during input fingerprinting

## Summary

Starting with **Spotless 8.6.0**, the `groovyGradle { greclipse() }` task fails with:

```
If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null, and neither was
```

This is a **regression**: **8.4.0 and 8.5.0 work** with the exact same project/config/Gradle version; **8.6.0 fails**. Pinning back to 8.5.0 is our current workaround.

It is closely related to (and may be the same root cause as) #2927 and #2502, but we can pinpoint it to the 8.5.0 → 8.6.0 bump, and it does **not** require the configuration cache (it happens with config cache disabled / not configured).

## Environment

- Spotless Gradle plugin **8.6.0** (8.4.0 and 8.5.0 OK)
- Gradle **9.5.1**
- JDK 26 (Temurin) — also reproduces on the same setup regardless
- Formatter: `groovyGradle { greclipse() }` (default greclipse → Groovy-Eclipse `5.7.0`/`e4.35`, Groovy `5.0.0`)
- Configuration cache: **not enabled**
- Build cache: disabled on the command line (`--no-build-cache`) — does **not** affect the outcome

## Spotless config (relevant part)

```groovy
spotless {
groovyGradle {
toggleOffOn()
greclipse()
leadingTabsToSpaces()
}
}
```

## How to reproduce

Run on a **clean Gradle user home** (e.g. a fresh CI container where greclipse is provisioned from scratch):

```
./gradlew spotlessCheck --no-daemon --no-build-cache --stacktrace
```

`:spotlessGroovyGradle` fails. It does **not** reproduce against a warm local cache where greclipse is already provisioned — which suggests the failure is tied to the freshly-built greclipse step state rather than to any formatting violation.

## Stack trace

The exception is thrown while Gradle **fingerprints the task's input properties** (before execution), when it calls `hashCode()` on Spotless's `ConfigurationCacheHackList`:

```
Caused by: java.lang.IllegalStateException: If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null, and neither was
at com.diffplug.spotless.FormatterStepSerializationRoundtrip.writeObject(FormatterStepSerializationRoundtrip.java:74)
at com.diffplug.spotless.FormatterStepSerializationRoundtrip$HackClone.writeObject(FormatterStepSerializationRoundtrip.java:121)
at com.diffplug.spotless.ConfigurationCacheHackList.writeObject(ConfigurationCacheHackList.java:78)
at com.diffplug.spotless.LazyForwardingEquality.toBytes(LazyForwardingEquality.java:110)
at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.lambda$serializedState$0(FormatterStepEqualityOnStateSerialization.java:95)
at com.diffplug.spotless.ThrowingEx.get(ThrowingEx.java:67)
at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.serializedState(FormatterStepEqualityOnStateSerialization.java:95)
at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.hashCode(FormatterStepEqualityOnStateSerialization.java:75)
at com.diffplug.spotless.FormatterStepSerializationRoundtrip$HackClone.hashCode(FormatterStepSerializationRoundtrip.java:142)
at com.diffplug.spotless.ConfigurationCacheHackList.hashCode(ConfigurationCacheHackList.java:151)
at org.gradle.internal.execution.impl.DefaultInputFingerprinter$InputCollectingVisitor.visitInputProperty(DefaultInputFingerprinter.java:131)
at org.gradle.api.internal.tasks.execution.TaskExecution.visitMutableInputs(TaskExecution.java:336)
at org.gradle.internal.execution.impl.DefaultInputFingerprinter.fingerprintInputProperties(DefaultInputFingerprinter.java:77)
at org.gradle.internal.execution.steps.CaptureMutableStateBeforeExecutionStep.captureExecutionStateWithOutputs(CaptureMutableStateBeforeExecutionStep.java:124)
...
at org.gradle.internal.execution.steps.LoadPreviousExecutionStateStep.executeMutable(LoadPreviousExecutionStateStep.java:36)
```

## Analysis / likely cause

- The failure is in `FormatterStepSerializationRoundtrip.writeObject` (line 74), which asserts that if `initializer == null` then one of `roundtripStateInternal` / `equalityStateInternal` must be non-null — and here **both are null**.
- It is reached via `ConfigurationCacheHackList.hashCode()` during Gradle's **input fingerprinting** (`DefaultInputFingerprinter` / `CaptureMutableStateBeforeExecutionStep`), i.e. before the task action runs — so it triggers for ordinary up-to-date checking, independent of the build cache and without the configuration cache.
- Because it only reproduces with a **freshly provisioned greclipse** (clean Gradle user home) and not against a warm cache, the greclipse step's state appears to be left without a computed `roundtripStateInternal`/`equalityStateInternal` (initializer already consumed) at the point Gradle serializes it for the fingerprint.

## Workaround

Pin the Spotless Gradle plugin to **8.5.0**.

Happy to provide a minimal reproducer repo or extra logs (`--info`/`--debug`) if helpful.

Start by reproducing the failure with a clean Gradle user home using ./gradlew spotlessCheck --no-daemon --no-build-cache --stacktrace. Read FormatterStepSerializationRoundtrip.java, ConfigurationCacheHackList.java, and the related serialization classes along the shown stack trace. Done means the freshly provisioned greclipse task no longer fails during input fingerprinting.

Scritto dal modello di indicizzazione a partire dal testo della issue.