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

[shulaoda]

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:

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

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 .".

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