fix(migrate): emit a single --check when rewriting prettier scripts (#2044)

#### Problem

When migrating a `package.json` script, prettier flags are rewritten to
`vp fmt`, converting `--list-different` / `-l` / `-c` to `--check`. If
the script also contained a literal `--check`, the result depended on
the flag order:

| input | before | after |
| --- | --- | --- |
| `prettier --check --list-different .` | `vp fmt --check .` | `vp fmt
--check .` |
| `prettier --list-different --check .` | `vp fmt --check --check .` ❌ |
`vp fmt --check .` |
| `prettier --check --check .` | `vp fmt --check --check .` ❌ | `vp fmt
--check .` |

`strip_flags_from_suffix` (in `script_rewrite.rs`) converts source flags
to the target flag and dedups against a literal `dedup_flag`. But the
dedup branch only set `conversion_emitted` — it did not drop the literal
token or `break`, so a literal `--check` that came **after** a converted
flag still fell through to `suffix.0.push(item)` and was emitted a
second time. Only the reverse order (`--check` first) worked, because
the literal was pushed once and the later conversion was suppressed.

#### Fix

Make the dedup branch symmetric — when the token is the literal
`dedup_flag`:

```rust
if val == conv.dedup_flag {
    // Drop a duplicate if the target flag was already emitted;
    // otherwise keep this one and mark it emitted.
    if conversion_emitted[ci] {
        converted = true;
    } else {
        conversion_emitted[ci] = true;
    }
    break;
}
```

Now the target flag is emitted exactly once regardless of order. The
added `break` is safe: prettier is the only config with a
`flag_conversions` rule and it has exactly one (eslint's is empty), so
nothing later in the loop is skipped.

#### Test

Extended `test_rewrite_prettier_list_different_to_check` with the two
previously-broken orderings — `--list-different --check` and the
double-literal `--check --check` — both asserting a single `vp fmt
--check .`. They fail on the old code (duplicate `--check`) and pass on
the fix. The full `vite_migration` suite stays green (293 passed).

#### Reproduce

```bash
cargo test -p vite_migration --lib \
  prettier::tests::test_rewrite_prettier_list_different_to_check
```

To see the bug: revert the `dedup_flag` branch to
`conversion_emitted[ci] = true;` (no `break`) and rerun — the test fails
with `left: "vp fmt --check --check ."`.

Co-authored-by: MK (fengmk2) <fengmk2@gmail.com>

Author: shulaoda

crates/vite_migration/src/prettier.rs

@@ -225,6 +225,15 @@ mod tests {
             rewrite_prettier_script("prettier --check --list-different ."),
             "vp fmt --check ."
         );
+
+        // --list-different + --check (the other order) → still a single --check
+        assert_eq!(
+            rewrite_prettier_script("prettier --list-different --check ."),
+            "vp fmt --check ."
+        );
+
+        // two literal --check → still a single --check
+        assert_eq!(rewrite_prettier_script("prettier --check --check ."), "vp fmt --check .");
     }
 
     #[test]

crates/vite_migration/src/script_rewrite.rs

@@ -497,7 +497,14 @@ fn strip_flags_from_suffix(
                     break;
                 }
                 if val == conv.dedup_flag {
-                    conversion_emitted[ci] = true;
+                    // Drop a duplicate if the target flag was already emitted;
+                    // otherwise keep this one and mark it emitted.
+                    if conversion_emitted[ci] {
+                        converted = true;
+                    } else {
+                        conversion_emitted[ci] = true;
+                    }
+                    break;
                 }
             }
             if converted {