feat: add agent_session_stopped and agent_session_title_changed events

[zimeg]

Adds Events API support for the two agent session events an agent app receives, mirroring the existing assistant event pattern (event model + payload wrapper + handler base class per event).

Events

• agent_session_stopped — the user clicked the native stop button while the agent session was processing.
  • type, channel, user, thread_ts, streaming_message_ts (list), event_ts
• agent_session_title_changed — the user renamed the session.
  • type, channel, user, team_id, thread_ts, title, previous_title, event_ts

What's included

• Event models: AgentSessionStoppedEvent, AgentSessionTitleChangedEvent (slack-api-model)
• Payload wrappers: AgentSessionStoppedPayload, AgentSessionTitleChangedPayload (slack-app-backend)
• Handler base classes: AgentSessionStoppedHandler, AgentSessionTitleChangedHandler
• Deserialization tests for both models

No central registry to edit — event routing is by reflection over the event type, so app.event(AgentSessionStoppedEvent.class, handler) self-registers (consistent with the other event types).

Notes

• Field sets were confirmed against live events, not just the docs. The two events are intentionally not symmetric: agent_session_title_changed carries team_id on the inner event; agent_session_stopped does not (it carries streaming_message_ts, the timestamps of any in-flight streaming messages at stop time). Envelope-level fields (enterprise_id, team_id on the wrapper, authorizations) live on the payload wrapper, not the inner event.
• Companion to the agent sessions methods (feat: add agents.sessions.setStatus and agents.sessions.rename #1633, merged) and the Node event types (feat(types): add agent_session_stopped and agent_session_title_changed events node-slack-sdk#2707).

Category

• slack-api-model / slack-app-backend

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.85%. Comparing base (b7562e3) to head (7017b90).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...end/events/handler/AgentSessionStoppedHandler.java	0.00%	2 Missing ⚠️
...vents/handler/AgentSessionTitleChangedHandler.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #1635 +/- ##
============================================
- Coverage 72.87% 72.85% -0.03% 
Complexity 4522 4522 ============================================
Files 479 481 +2 Lines 14390 14394 +4 Branches 1503 1503 ============================================
Hits 10487 10487 - Misses 3013 3017 +4 
Partials 890 890 

Flag	Coverage Δ
jdk-14	72.85% <0.00%> (-0.03%)	⬇️

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