Allow Java Format without Spotless

[FinlayRJW]

Before this PR

Applying the Java Format Gradle plugin without Spotless on the buildscript classpath fails on Gradle 9. PalantirJavaFormatSpotlessPlugin eagerly creates SpotlessInterop, so Gradle resolves its FormatterStep dependency even when no Spotless plugin is applied.

After this PR

SpotlessInterop is created only after both Java and Spotless are applied. Projects can use Java Format without putting Spotless on the buildscript classpath.

==COMMIT_MSG==
==COMMIT_MSG==

Testing

A regression test verifies that PalantirJavaFormatSpotlessPlugin can be applied when Spotless is absent. The existing Spotless compatibility matrix, Error Prone, formatting, and Checkstyle checks pass. Three affected Gradle 9.7 builds also configure successfully with the locally published fix.

Possible downsides?

None known. Spotless integration is initialised at the same point that it is used: after a supported Spotless plugin is applied.

Are Docs needed?

No. This restores the existing optional integration behaviour.

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Allow Java Format without Spotless

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