test: refactor mv illegal option tests to use test.each

Address code review feedback to reduce duplication by using parameterized tests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/shell/builtin/mv.zig

@@ -447,7 +447,7 @@ pub fn parseFlag(this: *Mv, flag: []const u8) union(enum) { continue_parsing, do
     if (flag[0] != '-') return .done;
 
     const small_flags = flag[1..];
-    for (small_flags) |char| {
+    for (small_flags, 0..) |char, i| {
         switch (char) {
             'f' => {
                 this.opts.force_overwrite = true;
@@ -471,7 +471,7 @@ pub fn parseFlag(this: *Mv, flag: []const u8) union(enum) { continue_parsing, do
                 this.opts.verbose_output = true;
             },
             else => {
-                return .{ .illegal_option = "-" };
+                return .{ .illegal_option = small_flags[i..][0..1] };
             },
         }
     }

test/regression/issue/26749.test.ts

@@ -0,0 +1,27 @@
+import { $ } from "bun";
+import { expect, test } from "bun:test";
+
+// https://github.com/oven-sh/bun/issues/26749
+// mv command should report the correct illegal option character in error messages
+
+const cases: [string, string][] = [
+  ["-T", "T"],
+  ["-X", "X"],
+  ["-fX", "X"],
+  ["-vZ", "Z"],
+];
+
+test.each(cases)("mv reports correct illegal option for %s", async (flag, expectedChar) => {
+  $.throws(true);
+  try {
+    await Bun.$`mv ${flag} ./a ./b`;
+    expect.unreachable("should have thrown");
+  } catch (e: unknown) {
+    const err = e as Bun.$.ShellError;
+    const stderr = new TextDecoder().decode(err.stderr);
+    expect(stderr).toContain(`mv: illegal option -- ${expectedChar}`);
+    expect(err.exitCode).not.toBe(0);
+  } finally {
+    $.nothrow();
+  }
+});