[2.x] fix: Prefer direct key selection for testFull at root (#8775)

**Problem**
`testFull` can select an aggregate-only key before a directly defined root key.
In issue #8772 this causes root project tests to be skipped, so a failing root test can pass unexpectedly.

**Solution**
Prefer directly defined keys in `Act.select`, and only fall back to aggregate-only candidates when no direct key exists.
Add scripted regression test `tests/i8772-root-project-testfull` and keep `project/extra-projects-key-aggregate` behavior intact.

main/src/main/scala/sbt/internal/Act.scala

@@ -216,7 +216,9 @@ object Act {
         case None    => noValidKeys
         case Some(x) => success(x)
       val validFilter = structure.fold(isValid(data))(isValidForAggregate(data, _))
-      selectFromValid(ss filter validFilter, default)
+      val valid = ss filter validFilter
+      val directlyDefined = valid filter isValid(data)
+      selectFromValid(if directlyDefined.nonEmpty then directlyDefined else valid, default)
     }
 
   private def isValidForAggregate(

sbt-app/src/sbt-test/tests/i8772-root-project-testfull/build.sbt

@@ -0,0 +1,11 @@
+ThisBuild / scalaVersion := "2.13.18"
+
+lazy val a = project
+
+lazy val b = project
+  .in(file("."))
+  .settings(
+    libraryDependencies += "org.scalatest" %% "scalatest-funsuite" % "3.2.19" % Test
+  )
+  .dependsOn(a)
+  .aggregate(a)

sbt-app/src/sbt-test/tests/i8772-root-project-testfull/src/test/scala/Test1.scala

@@ -0,0 +1,7 @@
+package example
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class Test1 extends AnyFunSuite {
+  fail("This test should fail")
+}

sbt-app/src/sbt-test/tests/i8772-root-project-testfull/test

@@ -0,0 +1 @@
+-> testFull