When send() fails due to a network error (ConnectError, ReadTimeout, etc.),
the failure was silently logged and the user received no feedback — appearing
as a hang. In one reported case, a user waited 1+ hour for a response that
had already been generated but failed to deliver (#2910).
Adds _send_with_retry() to BasePlatformAdapter:
- Transient errors: retry up to 2x with exponential backoff + jitter
- On exhaustion: send delivery-failure notice so user knows to retry
- Permanent errors: fall back to plain-text version (preserves existing behavior)
- SendResult.retryable flag for platform-specific transient errors
All adapters benefit automatically via BasePlatformAdapter inheritance.
Cherry-picked from PR #3108 by Mibayy.
Co-authored-by: Mibayy <mibayy@users.noreply.github.com>
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