parser: add infix, fn trailing comma, enum, struct literal tests

Port tests from upstream parser_test.zig:
- "respect line breaks after infix operators"
- "fn decl with trailing comma"
- "enum decl with no trailing comma"
- "struct literal no trailing comma"
- "2nd arg multiline string"
- "final arg multiline string"
- "function call with multiline argument"

No new parser.c changes needed — all features already implemented.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser_test.zig

@@ -1496,3 +1496,101 @@ test "zig fmt: spaces around slice operator" {
         \\
     );
 }
+
+test "zig fmt: respect line breaks after infix operators" {
+    try testCanonical(
+        \\comptime {
+        \\    self.crc =
+        \\        lookup_tables[0][p[7]] ^
+        \\        lookup_tables[1][p[6]] ^
+        \\        lookup_tables[2][p[5]] ^
+        \\        lookup_tables[3][p[4]] ^
+        \\        lookup_tables[4][@as(u8, self.crc >> 24)] ^
+        \\        lookup_tables[5][@as(u8, self.crc >> 16)] ^
+        \\        lookup_tables[6][@as(u8, self.crc >> 8)] ^
+        \\        lookup_tables[7][@as(u8, self.crc >> 0)];
+        \\}
+        \\
+    );
+}
+
+test "zig fmt: fn decl with trailing comma" {
+    try testTransform(
+        \\fn foo(a: i32, b: i32,) void {}
+    ,
+        \\fn foo(
+        \\    a: i32,
+        \\    b: i32,
+        \\) void {}
+        \\
+    );
+}
+
+test "zig fmt: enum decl with no trailing comma" {
+    try testTransform(
+        \\const StrLitKind = enum {Normal, C};
+    ,
+        \\const StrLitKind = enum { Normal, C };
+        \\
+    );
+}
+
+test "zig fmt: struct literal no trailing comma" {
+    try testTransform(
+        \\const a = foo{ .x = 1, .y = 2 };
+        \\const a = foo{ .x = 1,
+        \\    .y = 2 };
+        \\const a = foo{ .x = 1,
+        \\    .y = 2, };
+    ,
+        \\const a = foo{ .x = 1, .y = 2 };
+        \\const a = foo{ .x = 1, .y = 2 };
+        \\const a = foo{
+        \\    .x = 1,
+        \\    .y = 2,
+        \\};
+        \\
+    );
+}
+
+test "zig fmt: 2nd arg multiline string" {
+    try testCanonical(
+        \\comptime {
+        \\    cases.addAsm("hello world linux x86_64",
+        \\        \\.text
+        \\    , "Hello, world!\n");
+        \\}
+        \\
+    );
+    try testCanonical(
+        \\comptime {
+        \\    cases.addAsm("hello world linux x86_64",
+        \\        \\.text
+        \\    , "Hello, world!\n", "Hello, world!\n");
+        \\}
+        \\
+    );
+}
+
+test "zig fmt: final arg multiline string" {
+    try testCanonical(
+        \\comptime {
+        \\    cases.addAsm("hello world linux x86_64", "Hello, world!\n",
+        \\        \\.text
+        \\    );
+        \\}
+        \\
+    );
+}
+
+test "zig fmt: function call with multiline argument" {
+    try testCanonical(
+        \\comptime {
+        \\    self.user_input_options.put(name, UserInputOption{
+        \\        .name = name,
+        \\        .used = false,
+        \\    });
+        \\}
+        \\
+    );
+}