Add --dnd alias for --disable-nextest-doctest flag (mitsuhiko#822)

## Summary

Adds a shorter `--dnd` alias for the `--disable-nextest-doctest` flag to
make it easier to silence the deprecation warning during the transition
period.

This addresses the request from @ilyagr in mitsuhiko#803:
> It seems like people will pass this flag often to avoid the
deprecation warning. Perhaps give it a short version, e.g. `-D`?

After discussion, we agreed on `--dnd` as the alias.

## Changes

- Add `alias = "dnd"` to the `disable_nextest_doctest` CLI argument
- Update deprecation warning message to mention the `--dnd` alias: `Pass
'--disable-nextest-doctest' (or '--dnd') to update to this behavior now
and silence this warning.`
- Add test `test_nextest_doctest_dnd_alias_no_warning()` to verify the
alias works correctly

## Test plan

- [x] All existing tests pass (5/5 in nextest_doctest module)
- [x] New test verifies `--dnd` alias works correctly
- [x] Lints pass (cargo fmt, cargo clippy)
- [x] Manual verification: `cargo insta test --test-runner nextest
--dnd` works as expected

## Usage

Both forms work identically:
```bash
cargo insta test --test-runner nextest --disable-nextest-doctest
cargo insta test --test-runner nextest --dnd
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

cargo-insta/src/cli.rs

@@ -238,7 +238,7 @@ struct TestCommand {
     #[arg(long, hide = true)]
     no_force_pass: bool,
     /// Disable running doctests when using nextest test runner
-    #[arg(long)]
+    #[arg(long, alias = "dnd")]
     disable_nextest_doctest: bool,
     #[command(flatten)]
     target_args: TargetArgs,
@@ -897,7 +897,7 @@ fn test_run(mut cmd: TestCommand, color: ColorWhen) -> Result<(), Box<dyn Error>
         if has_doctests(&loc.packages) {
             eprintln!(
                 "{}: insta won't run a separate doctest process when using nextest in the future. \
-                 Pass `--disable-nextest-doctest` to update to this behavior now and silence this warning.",
+                 Pass `--disable-nextest-doctest` (or `--dnd`) to update to this behavior now and silence this warning.",
                 style("warning").bold().yellow()
             );
         }

cargo-insta/tests/functional/nextest_doctest.rs

@@ -46,7 +46,7 @@ fn test_simple() {
         "Expected warning message not found in stderr:\n{stderr}"
     );
     assert!(
-        stderr.contains("Pass `--disable-nextest-doctest` to update to this behavior now and silence this warning"),
+        stderr.contains("Pass `--disable-nextest-doctest` (or `--dnd`) to update to this behavior now and silence this warning"),
         "Expected flag suggestion not found in stderr:\n{stderr}"
     );
 }
@@ -181,3 +181,46 @@ fn test_simple() {
         "Warning should not appear with cargo-test runner:\n{stderr}"
     );
 }
+
+/// Test that nextest with --dnd alias doesn't show warning
+#[test]
+fn test_nextest_doctest_dnd_alias_no_warning() {
+    let test_project = TestFiles::new()
+        .add_cargo_toml("test_nextest_doctest_dnd_alias")
+        .add_file(
+            "src/lib.rs",
+            r#"
+/// This is a function with a doctest
+///
+/// ```
+/// assert_eq!(test_nextest_doctest_dnd_alias::add(2, 2), 4);
+/// ```
+pub fn add(a: i32, b: i32) -> i32 {
+    a + b
+}
+
+#[test]
+fn test_simple() {
+    insta::assert_snapshot!("test_value", @"test_value");
+}
+"#
+            .to_string(),
+        )
+        .create_project();
+
+    // Run with nextest and the --dnd alias, capture stderr to verify no warning
+    let output = test_project
+        .insta_cmd()
+        .args(["test", "--test-runner", "nextest", "--dnd", "--accept"])
+        .stderr(Stdio::piped())
+        .output()
+        .unwrap();
+
+    assert!(output.status.success());
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        !stderr.contains("warning: insta won't run a separate doctest process"),
+        "Warning message should not appear when --dnd alias is used:\n{stderr}"
+    );
+}