[build] de-noise output: defer DART_HOME check until test execution

[pq]

The DART_HOME check was happening eagerly during the configuration step which a spam warning when building the plugin. This fixes that.

Warning for Tests

☁  third_party [main] ⚡  ./gradlew :test --tests "com.jetbrains.dart.analysisServer.*"
Calculating task graph as no cached configuration is available for tasks: :test --tests com.jetbrains.dart.analysisServer.*

> Configure project :
plugin version: valueof(GradlePropertyValueSource)
ideaVersion: extension 'ideaVersion'
Following 1 plugins could not be created: plugins/fullLine/lib/modules/intellij.fullLine.yaml.jar

> Task :compileTestJava
Note: /Users/pquitslund/src/repos/dart-intellij-third-party/third_party/src/test/java/com/jetbrains/dart/analysisServer/DartServerHighlightingTest.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: /Users/pquitslund/src/repos/dart-intellij-third-party/third_party/src/test/java/com/jetbrains/lang/dart/highlighting/DartHighlightingTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

> Task :test
DART_HOME environment variable is not set. Dart Analysis Server tests will fail.

Not for Buiilds

☁  third_party [build_fixProdVersion] ⚡  ./gradlew buildPlugin -Pdev
Calculating task graph as configuration cache cannot be reused because the set of Gradle properties has changed: 'dev' was added.

> Configure project :
plugin version: 504.0.0-dev.20260331
ideaVersion: extension 'ideaVersion'
Following 1 plugins could not be created: plugins/fullLine/lib/modules/intellij.fullLine.yaml.jar

BUILD SUCCESSFUL in 5s
15 actionable tasks: 8 executed, 3 from cache, 4 up-to-date
Configuration cache entry stored.

---

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• See our [contributor guide](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md for general expectations for PRs.
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[gemini-code-assist]

Code Review

This pull request modifies the test task configuration in third_party/build.gradle.kts by wrapping the Dart SDK path validation and JVM argument setup within a doFirst block to ensure it only runs during task execution. A review comment points out that modifying task configuration inside doFirst is discouraged in modern Gradle because it interferes with the Configuration Cache and task up-to-date checks. The reviewer suggests using the providers API to lazily set the system property while keeping only the validation logging inside doFirst.

[pq]

WIP apparently!