feat(telegram): stream agent replies by editing the placeholder message in place (#3534)

* docs(spec): telegram streaming output design

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs(plan): telegram streaming implementation plan

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(telegram): report streaming support for telegram channel

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test(channels): use slack as the non-streaming sample channel in manager tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(telegram): register running-reply placeholder as stream target

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test(telegram): pin last_edit_at sentinel in placeholder registration test

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* refactor(telegram): extract _send_new_message from send()

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(telegram): edit streamed message in place for non-final updates

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(telegram): finalize streamed message with overflow splitting

When is_final=True arrives and stream state exists, pop the state, edit
the streamed placeholder with the final text, split overflow into follow-up
send_message calls, update _last_bot_message, and clear stream state.
Falls back to _send_new_message when no stream state is registered.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test(telegram): exercise the not-modified handler in final edit path

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs: telegram channel now streams replies via message editing

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(telegram): harden final-delivery path with guarded retry and chunk retries

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(channels): accept runtime 'messages' SSE event for streaming text accumulation

The embedded runtime (matching LangGraph Platform semantics) emits SSE
event name 'messages' for the requested 'messages-tuple' stream mode,
so the manager never accumulated token deltas and streaming channels
only updated from end-of-step 'values' snapshots — on Telegram this
looked like 'Working on it...' followed by the full answer in one block.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(telegram): widen stream-edit throttle to 3s in group chats

Telegram caps bots at 20 messages/minute per group, stricter than the
1 msg/s per-chat guideline. Groups have negative chat ids, so pick the
interval by sign.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(telegram): address review findings — thread fallback messages, bound stream registry, share stream-event constants

- Fallback/new stream messages now carry reply_to_message_id parsed from
  thread_ts so they stay nested under the user's message (finding 1)
- STREAM_MODES / MESSAGE_STREAM_EVENTS constants link the requested
  stream modes to the SSE event names they arrive under (finding 2)
- _register_stream_message bounds the in-flight registry at 256 entries,
  evicting oldest, guarding against leaks when a final never arrives (finding 4)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

backend/app/channels/manager.py

@@ -49,6 +49,11 @@ DEFAULT_RUN_CONTEXT: dict[str, Any] = {
     "subagent_enabled": False,
 }
 STREAM_UPDATE_MIN_INTERVAL_SECONDS = 0.35
+# Stream modes requested from the runtime, and the SSE event names under which
+# the message-tuple stream may arrive: the embedded runtime (and LangGraph
+# Platform) deliver the requested "messages-tuple" mode as event "messages".
+STREAM_MODES = ["messages-tuple", "values"]
+MESSAGE_STREAM_EVENTS = ("messages-tuple", "messages")
 THREAD_BUSY_MESSAGE = "This conversation is already processing another request. Please wait for it to finish and try again."
 
 CHANNEL_CAPABILITIES = {
@@ -56,7 +61,7 @@ CHANNEL_CAPABILITIES = {
     "discord": {"supports_streaming": False},
     "feishu": {"supports_streaming": True},
     "slack": {"supports_streaming": False},
-    "telegram": {"supports_streaming": False},
+    "telegram": {"supports_streaming": True},
     "wechat": {"supports_streaming": False},
     "wecom": {"supports_streaming": True},
 }
@@ -1135,7 +1140,7 @@ class ChannelManager:
             "input": {"messages": [human_message]},
             "config": run_config,
             "context": run_context,
-            "stream_mode": ["messages-tuple", "values"],
+            "stream_mode": list(STREAM_MODES),
             "multitask_strategy": "reject",
         }
         if owner_headers := _owner_headers(msg):
@@ -1150,7 +1155,7 @@ class ChannelManager:
                 event = getattr(chunk, "event", "")
                 data = getattr(chunk, "data", None)
 
-                if event == "messages-tuple":
+                if event in MESSAGE_STREAM_EVENTS:
                     accumulated_text, current_message_id = _accumulate_stream_text(streamed_buffers, current_message_id, data)
                     if accumulated_text:
                         latest_text = accumulated_text