test: cover the flag-off gating and non-enumerable completion cases

The completion bundle tested the flag-on behaviours but not two things worth
guaranteeing: that grammar completion stays suppressed when the experimental
engine is off (the gating contract), and that a non-enumerable next token
(number/regex) offers nothing rather than crashing or suggesting junk.

- GrammarValueCompletionTest.testFlagOffOffersNoGrammarCompletions: flag off, the
  AF_IN partial yields no AF_* names (the flag-on path's suggestions are absent).
- NextTokenChoicesTest.testNonEnumerableNextTokenOffersNothing: an IntegerTerminal
  port grammar returns an empty choice set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Steve Ramage

src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/completion/GrammarValueCompletionTest.kt

@@ -39,4 +39,14 @@ class GrammarValueCompletionTest : AbstractUnitFileTest() {
     val results = basicCompletionResultStrings
     assertContainsElements(results, "AF_INET", "AF_INET6")
   }
+
+  @Test
+  fun testFlagOffOffersNoGrammarCompletions() {
+    // Gating guarantee: with the experimental engine OFF (the default), grammar completion must not
+    // fire. A GrammarOptionValue has no legacy autocomplete options, so the same partial token that
+    // the flag-on path completes to AF_INET/AF_INET6 yields nothing grammar-derived here.
+    setupFileInEditor("file.service", "[Service]\nRestrictAddressFamilies=AF_IN${COMPLETION_POSITION}")
+
+    assertDoesntContain(basicCompletionResultStrings, "AF_INET", "AF_INET6")
+  }
 }

src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/NextTokenChoicesTest.kt

@@ -32,4 +32,11 @@ class NextTokenChoicesTest {
     // Context awareness: after a family and a separator, another family is expected.
     assertTrue(addressFamilies.nextTokenChoices("AF_INET ").contains("AF_INET6"))
   }
+
+  @Test
+  fun testNonEnumerableNextTokenOffersNothing() {
+    // Numbers/regexes/whitespace/EOF aren't enumerable, so there is nothing concrete to suggest.
+    val portGrammar = SequenceCombinator(IntegerTerminal(0, 65536), EOF())
+    assertTrue(portGrammar.nextTokenChoices("").isEmpty())
+  }
 }