Add ignore-change update regression coverage (#1782)

Author: Widthdom

changelog.d/unreleased/1782.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1782
+affected:
+  - src/CodeIndex/Cli/IndexCommandRunner.Update.cs
+  - tests/CodeIndex.Tests/IndexCommandRunnerTests.cs
+---
+
+## English
+
+- **Partial update ignore-file changes are now covered by regression tests (#1782)** - `--files` updates that include `.gitignore` or `.cdidxignore` fall back to a full scan so newly ignored stale rows are purged from the index.
+
+## 日本語
+
+- **部分更新で ignore file が変わった場合の挙動を回帰テストで固定しました (#1782)** - `.gitignore` や `.cdidxignore` を含む `--files` 更新は full scan に切り替わり、新たに ignore 対象になった古い行を index から削除します。

tests/CodeIndex.Tests/IndexCommandRunnerTests.cs

@@ -4529,6 +4529,41 @@ public void Run_FullScan_WithMalformedIgnoreRule_ReturnsSuccessWithWarningInstea
         }
     }
 
+    [Fact]
+    public void Run_UpdateMode_WhenIgnoreFileChanges_FallsBackToFullScanAndPurgesNowIgnoredRows()
+    {
+        var projectRoot = CreateTempProject();
+        try
+        {
+            File.WriteAllText(Path.Combine(projectRoot, "generated.py"), "print('generated')\n");
+            File.WriteAllText(Path.Combine(projectRoot, "keep.py"), "print('keep')\n");
+
+            var initialExitCode = IndexCommandRunner.Run([projectRoot, "--json"], _jsonOptions);
+            Assert.Equal(CommandExitCodes.Success, initialExitCode);
+
+            var dbPath = Path.Combine(projectRoot, ".cdidx", "codeindex.db");
+            Assert.Contains("generated.py", ReadIndexedPaths(dbPath));
+
+            File.WriteAllText(Path.Combine(projectRoot, ".gitignore"), "*.py\n!keep.py\n");
+
+            var (exitCode, json) = RunAndCaptureJson([projectRoot, "--files", ".gitignore", "--json"]);
+
+            Assert.Equal(CommandExitCodes.Success, exitCode);
+            Assert.Equal("success", json.GetProperty("status").GetString());
+            Assert.Equal("incremental", json.GetProperty("mode").GetString());
+
+            var indexedPaths = ReadIndexedPaths(dbPath);
+            Assert.DoesNotContain("generated.py", indexedPaths);
+            Assert.Contains("keep.py", indexedPaths);
+            Assert.Contains(".gitignore", indexedPaths);
+        }
+        finally
+        {
+            SqliteConnection.ClearAllPools();
+            DeleteDirectory(projectRoot);
+        }
+    }
+
     [Fact]
     public void Run_FullScan_SubdirectoryProjectRoot_UsesRepositoryIgnoreCaseConfigWhenTrue()
     {