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 72.84%. Comparing base (1a14eb4) to head (5ea65c2).
✅ 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     72.86%   72.84%   -0.03%     
+ Complexity     4534     4533       -1     
============================================
  Files           480      482       +2     
  Lines         14418    14422       +4     
  Branches       1512     1512              
============================================
- Hits          10506    10505       -1     
- Misses         3017     3022       +5     
  Partials        895      895              

Flag	Coverage Δ
jdk-14	72.84% <0.00%> (-0.03%)	⬇️

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[zimeg]

⚡ Will merge to match documented events!