feat(telegram): streaming output support (Edit + Draft modes)

[Youhai020616]

Summary

• Add progressive streaming output for the Telegram adapter so AI responses display incrementally ("typing effect") instead of waiting for the full response
• Implement two streaming strategies: Edit mode (sendMessage + editMessageText, works in groups and DMs) and Draft mode (sendMessageDraft Bot API 9.5, DM only, experimental)
• Auto mode (default) selects the best strategy; stream_mode: off preserves existing behavior with zero impact

Changes

File	What
openfang-types/config.rs	TelegramStreamMode enum, TelegramStreamConfig struct, new fields on TelegramConfig
openfang-channels/types.rs	ChannelStreamEvent enum, StreamSink trait, supports_streaming() / begin_stream() on ChannelAdapter
openfang-channels/formatter.rs	escape_html_entities(), find_split_point() helpers
openfang-channels/telegram.rs	TelegramStreamSink implementing StreamSink, Edit/Draft modes, rate-limit backoff, message splitting, cursor indicator
openfang-channels/bridge.rs	send_message_streaming() on ChannelBridgeHandle, dispatch_streaming(), streaming fallback in dispatch
openfang-api/channel_bridge.rs	send_message_streaming() impl with StreamEvent → ChannelStreamEvent conversion

+819 lines, -19 lines across 7 files (including Cargo.lock)

Key Features

• Dual-threshold buffering: flushes when both time (500ms) AND character count (40 chars) thresholds are met
• Cursor indicator ▌ shown during streaming, removed on finalize
• 429 rate-limit adaptive backoff: doubles flush interval after 3 consecutive rate limits
• Auto message splitting at 3900 chars with smart break points (paragraph > line > sentence > word)
• Draft → Edit fallback: if Draft mode fails, automatically degrades to Edit
• Full backward compatibility: all trait changes use default implementations

Config Example

[channels.telegram]
stream_mode = "auto"  # off | edit | draft | auto

[channels.telegram.stream_config]
flush_interval_ms = 500
min_chars_per_flush = 40
max_message_chars = 3900
cursor_indicator = "▌"

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