Play Framework

perf(utils): migrate AlternativeDateFormat to thread-safe DateTimeFormatter

Replaces the synchronized SimpleDateFormat list with an immutable
DateTimeFormatter list, removing the global monitor that previously
serialized every date parse across the whole app. Throughput is now
bounded by per-thread parse latency rather than by one parse at a time.

Behavior preservation:
- ResolverStyle.LENIENT keeps the SDF rollover semantics callers rely
  on (e.g., month overflow, day overflow). Existing parse outcomes are
  unchanged for the 13 default patterns.
- ZoneId captured once at construction mirrors SDF's "zone fixed at
  formatter creation" contract; later TimeZone.setDefault() does not
  retroactively shift parses through this formatter.
- setFormats() retains its existing append (not replace) semantics.

Concurrency:
- DateTimeFormatter is documented immutable + thread-safe, so parse()
  takes no monitor.
- The compiled-pattern list is volatile, so a parse() iterating the
  list never sees a partially-published setFormats() update.

Tests:
- Adds 10 new UtilsTest cases exercising every default parse path
  (ISO8601 with/without literal Z, space-separated date-time, date-only,
  compact ddMMyyyy, slashed and dashed European formats, length-mismatch
  rejection, unrecognized-input rejection, setFormats-append).
- Two latent quirks surfaced by the test scaffolding are preserved
  rather than silently changed: yyyyMMdd is not a default pattern, and
  dd-MM-yyyy HH:mm:ss is unreachable under current ordering because
  yyyy-MM-dd HH:mm:ss lenient eats the input first. Worth a separate
  format-ordering fix; explicitly out of scope here.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>