fix(customParseFormat): preserve UTC mode when format is an array

[Jadu07]

What this PR does

Fixes #3013 — dayjs.utc() ignores UTC mode when the format argument is an array.

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The bug

If you pass an array of formats to dayjs.utc() (via the customParseFormat plugin), the date gets parsed in local time instead of UTC. No error is thrown, which makes the bug really hard to spot — your tests might pass fine in TZ=UTC but silently break everywhere else.

dayjs.extend(utc)
dayjs.extend(customParseFormat)

// single format — works fine
dayjs.utc('1995-05-01', 'YYYY-MM-DD').toISOString()
// → '1995-05-01T00:00:00.000Z' ✓

// array format — parsed in local time before this fix
dayjs.utc('1995-05-01', ['YYYY-MM-DD', 'YYYY-M-D']).toISOString()
// → '1995-05-01T23:00:00.000Z' in UTC+1 ✗

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Why it happened

In customParseFormat/index.js, the format instanceof Array branch loops through each format and tries it with d.apply(this, args). The problem is that d.apply just calls the regular dayjs() constructor, which has no idea about utc: true. So every format attempt ends up using local time.

Meanwhile, the single-format code path calls parseFormattedInput(date, format, utc, d), which correctly threads the utc flag through.

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The fix

One line change — once we find a valid format, compute this.$d using parseFormattedInput with the utc flag instead of blindly copying result.$d:

  if (result.isValid()) {
-   this.$d = result.$d
+   this.$d = utc
+     ? parseFormattedInput(date, format[i - 1], utc, d)
+     : result.$d

---

Tests

Added two regression tests to test/plugin/utc.test.js:

• first format in the array matches → result is UTC midnight ✓
• second format in the array matches → result is UTC midnight ✓

Full suite: 774 tests, 93 suites — all passing. npm run lint is clean too.

[Jadu07]

@iamkun @pavan-sh pls review this pr.