fix(gateway): stop typing indicator on Telegram after session completion

[vincent-wen789]

Summary

Fixes stuck typing indicator on Telegram that persists after normal session completion.

Problem

Two independent issues caused Telegram typing to stay visible indefinitely:

1. Normal completion: _keep_typing is cancelled, but Telegram adapter had no stop_typing() implementation — the base class no-op left the typing bubble to expire on its own (~5s). Some Telegram clients cache the indicator state and never clear it.

2. Dead-session timeout: cancel_session_processing gives up after 5s if the task is stuck in blocking I/O. Without the chat_id parameter, the interrupt event was never signaled, so _keep_typing kept refreshing the typing bubble every 2 seconds indefinitely.

Changes

gateway/platforms/telegram.py

• Add stop_typing() implementation that sends choose_sticker chat action to supersede the typing action (Telegram has no explicit "cancel typing" API)
• Non-fatal — if the API call fails, the bubble still expires within 5s once _keep_typing stops refreshing

gateway/platforms/base.py

• Add optional chat_id parameter to cancel_session_processing
• On timeout path (_gave_up), call interrupt_session_activity to signal the interrupt event and invoke stop_typing on the adapter
• Update the sole call site in _handle_slash_command to pass chat_id
• Reorder command handling: send response BEFORE cancelling old task (race condition fix from issue Race condition: /new during active agent session never sends response (Telegram gateway) #18912)

Test Plan

• Verified typing indicator clears after normal DM session completion (manual test on Telegram)
• Gateway restart with changes confirmed stable

Related

• Issue Race condition: /new during active agent session never sends response (Telegram gateway) #18912 (race condition: /new response dropped during active agent)
• Similar dead-session typing issues: fix(install.ps1): drop UTF-8 BOM that breaks the irm | iex one-liner (#27397) #27419, feat: add BedrockTransport + wire all Bedrock transport paths #27075

[alt-glitch]

This PR appears to supersede #20696 — the base.py changes are identical, and this PR adds the Telegram-specific stop_typing() implementation on top. Consider closing #20696 if this one is merged.

Related PRs addressing typing indicator issues: #19278, #20780.

[teknium1]

Thanks for tracing the dead-session typing path. The timeout case still needs a fix, but this patch cannot land unchanged.

Problems

• The added interrupt_session_activity() call would target the wrong event. _dispatch_active_session_command replaces self._active_sessions[session_key] with command_guard before it calls cancel_session_processing (PR-base gateway/platforms/base.py:2485-2487). interrupt_session_activity resolves its event from that mapping (gateway/platforms/base.py:3910-3913), while _keep_typing retains the original event as stop_event (gateway/platforms/base.py:4825,4841-4846).
• choose_sticker is a Telegram activity action for an imminent sticker, not a cancellation action; it would show a different activity state rather than clear one.
• Please add a regression test for a task that survives cancellation past the timeout. Current cleanup coverage only protects the normal refresh-cancellation path (tests/gateway/test_keep_typing_timeout.py:203-236).

Suggested changes

• Preserve the original session event before installing command_guard, set that event on the timeout path, and use current main's _stop_typing_refresh cleanup rather than a Telegram-specific replacement action.

Automated hermes-sweeper review.