Gate test-runner injection in ScopedBuildInfo behind a flag

Only inject the JVM test-runner dependency into the Test scope when
generating BuildInfo for export --json. The runtime BuildInfo.scala
generator (ScopedSources.buildInfo) must not inject it: scala-cli
supplies the test-runner at runtime, so users' BuildInfo.Test.dependencies
would otherwise contain a dep they never declared, breaking
RunTestsDefault."BuildInfo fields should be reachable".

Author: kubukoz

modules/cli/src/main/scala/scala/cli/exportCmd/JsonProjectDescriptor.scala

@@ -20,7 +20,13 @@ final case class JsonProjectDescriptor(
       val sourcePaths   = sources.paths.map(_._1.toString)
       val inMemoryPaths = sources.inMemory.flatMap(_.originalPath.toSeq.map(_._2.toString))
 
-      ScopedBuildInfo.forScope(options, sourcePaths ++ inMemoryPaths, scope, logger)
+      ScopedBuildInfo.forScope(
+        options,
+        sourcePaths ++ inMemoryPaths,
+        scope,
+        logger,
+        injectTestRunner = true
+      )
 
     for {
       baseBuildInfo <- BuildInfo(optionsMain, workspace)

modules/options/src/main/scala/scala/build/info/ScopedBuildInfo.scala

@@ -62,9 +62,11 @@ object ScopedBuildInfo {
     )
       .reduceLeft(_ + _)
 
-  /** Build a [[ScopedBuildInfo]] for [[scope]] and inject the JVM test-runner dependency that
-    * scala-cli silently adds at test time, so consumers of `export --json` see the same classpath
-    * scala-cli would use.
+  /** Build a [[ScopedBuildInfo]] for [[scope]]. When [[injectTestRunner]] is true, also inject the
+    * JVM test-runner dependency that scala-cli silently adds at test time, so consumers of
+    * `export --json` see the same classpath scala-cli would use. The runtime `BuildInfo.scala`
+    * generator must leave it false — the test-runner is supplied by scala-cli at runtime, not
+    * declared by the user.
     *
     * Injection conditions match [[scala.build.Artifacts.apply]]: scope is Test, the scope is
     * non-empty (has sources), the platform is JVM, and the build has a Scala version (i.e. is not
@@ -74,10 +76,11 @@ object ScopedBuildInfo {
     options: BuildOptions,
     sourcePaths: Seq[String],
     scope: Scope,
-    logger: Logger
+    logger: Logger,
+    injectTestRunner: Boolean = false
   ): ScopedBuildInfo = {
     val base = apply(options, sourcePaths)
-    if scope == Scope.Test && sourcePaths.nonEmpty then
+    if injectTestRunner && scope == Scope.Test && sourcePaths.nonEmpty then
       withJvmTestRunner(base, options, logger)
     else base
   }