Move message splitting from individual channels (Discord) to the Manager
layer via per-channel worker goroutines. Each channel now declares its
max message length through BaseChannelOption/MessageLengthProvider, and
the Manager automatically splits oversized outbound messages before
dispatch. This prevents one slow channel from blocking all others.
- Add WithMaxMessageLength option and MessageLengthProvider interface
- Set platform-specific limits (Discord 2000, Telegram 4096, Slack 40000, etc.)
- Convert SplitMessage to rune-aware counting for correct Unicode handling
- Replace single dispatcher goroutine with per-channel buffered worker queues
- Remove Discord's internal SplitMessage call (now handled centrally)
- Accept hard upper limit (maxLen) instead of pre-subtracted value
- Caller now passes actual platform limit (e.g., 2000 for Discord)
- Internal buffer of 500 chars is handled within message.go
- Preferred split at maxLen - 500, may extend to maxLen for code blocks
- Never exceeds maxLen, no more mental math for callers
- Move FindLast, findLast, and SplitMessage from discord.go to pkg/utils/message.go
- Update discord.go to use utils.SplitMessage()
- Makes splitting logic reusable across other channels
Hoshina