fix(telegram): address Copilot review feedback

- Fix silent parse failures: message_id.parse().unwrap_or(0) replaced
  with match + tracing::warn on parse error (update_draft, finalize_draft)
- Fix UTF-8 panic: byte-based truncation replaced with char_indices()
  safe boundary detection for TELEGRAM_MAX_MESSAGE_LENGTH
- Fix global rate limiter: Mutex<Option<Instant>> replaced with
  Mutex<HashMap<String, Instant>> for per-chat rate limiting so
  concurrent conversations don't interfere with each other
- Document Block variant: clarify it's reserved for future use and
  currently behaves the same as Partial

src/config/schema.rs

@@ -1449,7 +1449,8 @@ pub enum StreamMode {
     Off,
     /// Update a draft message with every flush interval.
     Partial,
-    /// Update a draft message in larger chunks.
+    /// Update a draft message in larger chunks (reserved for future use;
+    /// currently behaves the same as `Partial`).
     Block,
 }