refactor: discord_timestamp delegates to discord.utils.format_dt

T3-21: the hand-rolled <t:unix:style> markup duplicated discord.py's
format_dt. style validation stays (dpy_commons' ValueError contract).
output is byte-identical for all 7 styles across naive (incl. DST fold/gap),
aware, pre-epoch and sub-second datetimes in three timezones.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_commons/text.py

@@ -7,6 +7,8 @@ import re
 from datetime import datetime
 from typing import Optional, Sequence
 
+from discord.utils import format_dt
+
 from .limits import MSG_LIMIT
 
 _ELLIPSIS = "…"
@@ -108,9 +110,7 @@ def discord_timestamp(dt: datetime, style: str = "f") -> str:
     aware dt if the source is UTC; raises ValueError on an invalid style (one of t/T/d/D/f/F/R)"""
     if style not in _TIMESTAMP_STYLES:
         raise ValueError(f"invalid timestamp style '{style}'; expected one of {_TIMESTAMP_STYLES}")
-    if dt.tzinfo is None:
-        dt = dt.astimezone()
-    return f"<t:{int(dt.timestamp())}:{style}>"
+    return format_dt(dt, style)
 
 
 def humanize_delta(seconds: float) -> str: