fix(discord): split-and-deliver oversized edits instead of silent truncation

[AhmetArif0]

Problem

DiscordAdapter.edit_message silently truncated messages that exceeded
the 2000-character limit:

if len(formatted) > self.MAX_MESSAGE_LENGTH:
    formatted = formatted[:self.MAX_MESSAGE_LENGTH - 3] + "..."
await msg.edit(content=formatted)
return SendResult(success=True, message_id=message_id)  # lies: tail was dropped

stream_consumer received success=True and assumed the full content
was on screen. Any streaming response that grew past 2000 chars had its
tail permanently dropped with no error surfaced to the user or the
gateway.

Fix

Replace the truncation with _edit_overflow_split(), mirroring the
same contract Telegram's overflow split established in PR #23576:

1. Edit the original message with chunk 1 (so the message stays
   in-thread and doesn't flash as "deleted + new").
2. Send remaining chunks as reply continuations to the previous
   chunk, so the user sees a contiguous threaded block.
3. Return SendResult(success=True, message_id=<last-chunk-id>, continuation_message_ids=(...)).

stream_consumer already handles continuation_message_ids
(line 1172) — it updates _message_id to the last continuation so
subsequent edits target the most recent visible message.

Behavior

Scenario	Before	After
Edit ≤ 2000 chars	Edit in place ✅	Edit in place ✅
Edit > 2000 chars	Silent truncation with "..." ❌	Split and deliver all chunks ✅
First-chunk edit fails (real error)	Logged, success=False	Logged, success=False ✅
First-chunk "not modified"	N/A	Continue to send continuations ✅
Continuation send fails	N/A	Best-effort: deliver what succeeded, log warning ✅

Checklist

• No new imports required (List, Any already imported from typing)
• truncate_message inherited from BasePlatformAdapter (same as send_message uses)
• to_reference(fail_if_not_exists=False) — same pattern as send_message's reply threading
• stream_consumer already consumes continuation_message_ids — no consumer changes needed
• No competitor PRs targeting this path

[teknium1]

Fixed on main via #55592 (commit af5cea0). You identified this same Discord edit_message truncation bug first (May 11) — credited and co-authored on the merged commit alongside #27961. The merged version gates the split on finalize to avoid the #48648 mid-stream duplication loop. Thanks!