Telegram flood control during streaming caused messages to be cut off
mid-response. The old behavior permanently disabled edits after a single
flood-control failure, losing the remainder of the response.
Changes:
- Adaptive backoff: on flood-control edit failures, double the edit interval
instead of immediately disabling edits. Only permanently disable after 3
consecutive failures (_MAX_FLOOD_STRIKES).
- Cursor strip: when entering fallback mode, best-effort edit to remove the
cursor (▉) from the last visible message so it doesn't appear stuck.
- Fallback send retry: _send_fallback_final retries each chunk once on
flood-control failures (3s delay) before giving up.
- Default edit_interval increased from 0.3s to 1.0s. Telegram rate-limits
edits at ~1/s per message; 0.3s was virtually guaranteed to trigger flood
control on any non-trivial response.
- _send_or_edit returns bool so the overflow split loop knows not to
truncate accumulated text when an edit fails (prevents content loss).
Fixes: messages cutting/stopping mid-response on Telegram, especially
with streaming enabled.
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