feat: event listener filter middleware for agents

[srtaalej]

Summary

This PR adds agentEventFilter(), a listener middleware that filters out events agents don't need — bot messages, slackbot, and all message subtypes (message_changed, message_deleted, channel_join, etc.)

• Works across listener types: app.message(), app.event('app_mention'), and any other event listener
• Configurable via allow and block options for fine-grained control over which subtypes pass through

Motivation

Agent developers building on Bolt currently have to write the same boilerplate in every event handler:

 app.message(async ({ message }) => {                                                                                                                                               
   if (message.subtype) return;                                                                                                                                                     
   if (message.bot_id) return;                                                                                                                                                      
   if (message.user === 'USLACKBOT') return;                                                                                                                                        
   // ...actual logic                                                                                                                                                               
 });                                                                                                                                                                                

This is error-prone and easy to get wrong. agentEventFilter() replaces all of that with a single middleware call.

Testing

 import { App, agentEventFilter } from '@slack/bolt';                                                                                                                               
                                                                                                                                                                                    
 // Default: only plain user messages pass through                                                                                                                                  
 app.message(agentEventFilter(), async ({ message, say }) => {                                                                                                                      
   await say(`You said: ${message.text}`);                                                                                                                                          
 });                                                                                                                                                                                
                                                                                                                                                                                    
 // Also allow file_share messages through                                                                                                                                          
 app.message(agentEventFilter({ allow: ['file_share'] }), async ({ message, say }) => {
   if (message.subtype === 'file_share') {                                                                                                                                          
     await say('Thanks for the file!');                                                                                                                                             
   }                                                                                                                                                                                
 });                                                                                                                                                                                
                                                                                                                                                                                    
 // Works with app_mention too                                                                                                                                                      
 app.event('app_mention', agentEventFilter(), async ({ event, say }) => {                                                                                                           
   await say('You mentioned me!');                                                                                                                                                  
 });   

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 98.63014% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.55%. Comparing base (8d10b03) to head (fc36140).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/middleware/builtin.ts	98.63%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2840      +/-   ##
==========================================
+ Coverage   93.50%   93.55%   +0.04%     
==========================================
  Files          42       42              
  Lines        7747     7820      +73     
  Branches      678      688      +10     
==========================================
+ Hits         7244     7316      +72     
- Misses        498      499       +1     
  Partials        5        5              

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