Assert the PSI wiring the ParserDefinition unblocked

JaiFileTypeTest could previously only check the virtual file's type, because
without a ParserDefinition the PSI file fell back to plain text. It now asserts
the PSI file is a JaiFile in the Jai language and that JaiParserDefinition is the
registered definition - the wiring every PSI-dependent feature needs.

Also records the in-memory-fixture flake ('Cannot create child file at /src') as
infrastructure, with the note that JaiCorpusParserTest sidesteps it by using
PsiFileFactory directly.

src/test/kotlin/dev/hgh/jai/JaiFileTypeTest.kt

@@ -1,7 +1,10 @@
 package dev.hgh.jai
 
+import com.intellij.lang.LanguageParserDefinitions
 import com.intellij.openapi.fileTypes.FileTypeManager
 import com.intellij.testFramework.fixtures.BasePlatformTestCase
+import dev.hgh.jai.psi.JaiFile
+import dev.hgh.jai.psi.JaiParserDefinition
 
 /**
  * Verifies plugin.xml wiring: the `.jai` extension resolves to our file type and
@@ -24,12 +27,24 @@ class JaiFileTypeTest : BasePlatformTestCase() {
     }
 
     /**
-     * The *virtual* file already resolves to our type. The PSI file's language stays
-     * plain text until a ParserDefinition is registered (phase 3) — asserting that here
-     * would be asserting the platform's fallback, not our wiring.
+     * Before phase 3 this could only assert the *virtual* file's type, because
+     * without a `ParserDefinition` the PSI file fell back to plain text. Now that
+     * `JaiParserDefinition` is registered, the PSI file itself is a [JaiFile] — which
+     * is what every PSI-dependent feature (structure view, folding, references)
+     * relies on.
      */
-    fun testVirtualFileUsesJaiFileType() {
+    fun testJaiFileProducesJaiPsi() {
         val file = myFixture.configureByText("sample.jai", "main :: () { }")
         assertEquals(JaiFileType, file.virtualFile.fileType)
+        assertEquals(JaiLanguage, file.language)
+        assertTrue("expected a JaiFile, got ${file.javaClass.name}", file is JaiFile)
+    }
+
+    fun testParserDefinitionIsRegisteredForJai() {
+        val definition = LanguageParserDefinitions.INSTANCE.forLanguage(JaiLanguage)
+        assertTrue(
+            "plugin.xml must register our parser definition, got $definition",
+            definition is JaiParserDefinition,
+        )
     }
 }