Fix indentation of if/else in 'csharp-ts-mode' (bug#70345)

* lisp/progmodes/csharp-mode.el (csharp-ts-mode--indent-rules):
Fix indentation rules for 'if' and 'else'.

* test/lisp/progmodes/csharp-ts-mode-tests.el:
* test/lisp/progmodes/csharp-ts-mode-resources/indent.erts: New
test files.

Copyright-paperwork-exempt: yes

lisp/progmodes/csharp-mode.el

@@ -689,7 +689,9 @@ compilation and evaluation time conflicts."
      ((parent-is "binary_expression") parent 0)
      ((parent-is "block") parent-bol csharp-ts-mode-indent-offset)
      ((parent-is "local_function_statement") parent-bol 0)
-     ((parent-is "if_statement") parent-bol 0)
+     ((match "block" "if_statement") parent-bol 0)
+     ((match "else" "if_statement") parent-bol 0)
+     ((parent-is "if_statement") parent-bol csharp-ts-mode-indent-offset)
      ((parent-is "for_statement") parent-bol 0)
      ((parent-is "for_each_statement") parent-bol 0)
      ((parent-is "while_statement") parent-bol 0)

test/lisp/progmodes/csharp-ts-mode-resources/indent.erts

@@ -0,0 +1,51 @@
+Code:
+  (lambda ()
+    (csharp-ts-mode)
+    (indent-region (point-min) (point-max)))
+
+Point-Char: |
+
+Name: Indent single statement body for if/else. (bug#70345)
+
+=-=
+
+int x;
+int y;
+
+if (true)
+    x = 2;
+
+if (true)
+{
+    x = 2;
+}
+
+if (true)
+    x = 2;
+else
+    y = 2;
+
+if (true)
+{
+    x = 2;
+}
+else
+{
+    y = 2;
+}
+
+if (true)
+    x = 2;
+else
+{
+    y = 2;
+}
+
+if (true)
+{
+    x = 2;
+}
+else
+    y = 2;
+
+=-=-=

test/lisp/progmodes/csharp-ts-mode-tests.el

@@ -0,0 +1,30 @@
+;;; csharp-ts-mode-tests.el --- Tests for Tree-sitter-based C# mode  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2024 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'ert-x)
+(require 'csharp-mode)
+
+(ert-deftest csharp-ts-mode-test-indentation ()
+  (ert-test-erts-file (ert-resource-file "indent.erts")))
+
+(provide 'csharp-ts-mode-tests)
+;;; csharp-ts-mode-tests.el ends here