Add a platform-neutral send-failure vocabulary so consumers can branch on a
typed category instead of substring-matching the raw provider message.
- base.py: SEND_ERROR_KINDS + classify_send_error() (too_long / bad_format /
forbidden / not_found / rate_limited / transient / unknown), and an optional
SendResult.error_kind field (defaults None — fully backward compatible).
- telegram.py: populate error_kind on send() failures; message_too_long keeps
its existing error token plus error_kind='too_long'.
Purely additive: no behavioral change to the existing degrade-and-deliver
paths (MarkdownV2->plain-text fallback, overflow split, retry classification
all untouched). 22 new tests + 210 adapter regression tests green.
