[Fix] Fast mode repeats loading reactions during active Slack conversations

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote or in the web UI.

What changed

• show Slack's temporary Fast-mode processing reaction when a conversation session is first created or explicitly re-entered with /fast or !fast
• skip that processing reaction for ordinary messages routed into an existing Fast conversation, including default Fast-mode channel handling
• preserve intentional emoji-only closeouts and the existing cleanup behavior when a processing reaction was actually added
• treat processing-reaction session lookups as best-effort so transient database failures still start Fast processing with visible feedback
• cover session creation, explicit re-entry, active-session continuation, lookup failures, route wiring, and emoji-only closeouts with focused regression tests

Why this change was made

Fast mode added the configured Slack loading reaction on every handled message, even after the conversation session already existed. Standard Roomote task handling reserves that acknowledgement for startup or wake transitions rather than routine delivery into active work.

Impact

Active Fast conversations no longer accumulate repetitive temporary :eyes: reactions on normal follow-ups. Explicit Fast commands still provide immediate wake feedback, and transient session lookup failures fall back to showing the reaction rather than dropping the Slack message. Fast mode does not introduce an idle timeout or a continuously running process to model session activity.

[roomote-community]

No new code issues found. See task

• apps/api/src/handlers/slack/events/message-entry.ts:1767: A failed best-effort session lookup drops default-Fast Slack messages before Fast processing starts.

_{Reviewed 957658b}

[roomote-roomote]

Fixed the unresolved session-lookup failure in 957658b2.

• Both new processing-reaction session lookups are now best-effort; transient failures log and fall back to showing :eyes: while Fast processing continues.
• Added regression coverage for the rejected lookup path.
• Focused validation passed: 3 test files / 22 tests, API typecheck, changed-file ESLint, Oxlint, and formatting.
• The full pre-push gate passed.
• Follow-up review found no new code issues, the review thread is resolved, and all 17 CI checks completed (14 successful, 3 skipped).