feat(channel): stream LLM responses to Telegram via draft message edits

Wire the existing provider-layer streaming infrastructure through the
channel trait and agent loop so Telegram users see tokens arrive
progressively via editMessageText, instead of waiting for the full
response.

Changes:
- Add StreamMode enum (off/partial/block) and draft_update_interval_ms
  to TelegramConfig (backward-compatible defaults: off, 1000ms)
- Add supports_draft_updates/send_draft/update_draft/finalize_draft to
  Channel trait with no-op defaults (zero impact on existing channels)
- Implement draft methods on TelegramChannel using sendMessage +
  editMessageText with rate limiting and Markdown fallback
- Add on_delta mpsc::Sender<String> parameter to run_tool_call_loop
  (None preserves existing behavior)
- Wire streaming in process_channel_message: when channel supports
  drafts, send initial draft, spawn updater task, finalize on completion

Edge cases handled:
- 4096-char limit: finalize draft and fall back to chunked send
- Broken Markdown: use no parse_mode during streaming, apply on finalize
- Edit failures: fall back to sending complete response as new message
- Rate limiting: configurable draft_update_interval_ms (default 1s)

src/config/mod.rs

@@ -9,7 +9,8 @@ pub use schema::{
     LarkConfig, MatrixConfig, MemoryConfig, ModelRouteConfig, ObservabilityConfig,
     PeripheralBoardConfig, PeripheralsConfig, QueryClassificationConfig, ReliabilityConfig,
     ResourceLimitsConfig, RuntimeConfig, SandboxBackend, SandboxConfig, SchedulerConfig,
-    SecretsConfig, SecurityConfig, SlackConfig, TelegramConfig, TunnelConfig, WebSearchConfig,
+    SecretsConfig, SecurityConfig, SlackConfig, StreamMode, TelegramConfig, TunnelConfig,
+    WebSearchConfig,
     WebhookConfig,
 };
 
@@ -31,6 +32,8 @@ mod tests {
         let telegram = TelegramConfig {
             bot_token: "token".into(),
             allowed_users: vec!["alice".into()],
+            stream_mode: StreamMode::default(),
+            draft_update_interval_ms: 1000,
         };
 
         let discord = DiscordConfig {