feat(socket-mode): replace error interfaces with proper Error subclasses

[WilliamBergamin]

Summary

This PR replaces the error factory-function pattern in @slack/socket-mode with proper Error subclasses, bringing it in line with the same change already applied to @slack/web-api in #2593.

What changed

The old approach used interfaces (e.g. SMWebsocketError, SMPlatformError) and factory functions (e.g. websocketErrorWithOriginal(), platformErrorFromEvent()) that mutated plain Error objects by attaching a code property. This made instanceof checks unreliable.

The new approach defines real ES6 class subclasses of Error:

• SMWebsocketError
• SMPlatformError
• SMNoReplyReceivedError
• SMSendWhileDisconnectedError
• SMSendWhileNotReadyError

How users can take advantage of these changes

Before (fragile string-based check):

 import { ErrorCode } from '@slack/socket-mode';

 client.on('error', (err) => {
   if (err.code === ErrorCode.WebsocketError) {
     console.log('WebSocket issue:', err.original?.message);
   }
 });

After (type-safe instanceof check):

 import { SMWebsocketError, SMSendWhileDisconnectedError } from '@slack/socket-mode';

 client.on('error', (err) => {
   if (err instanceof SMWebsocketError) {
     // TypeScript knows `err.original` and `err.cause` exist
     console.log('WebSocket issue:', err.original.message);
   }
 });

Catching send errors:

 import { SMSendWhileDisconnectedError, SMSendWhileNotReadyError } from '@slack/socket-mode';

 try {
   await client.send({ type: 'message', text: 'hello' });
 } catch (err) {
   if (err instanceof SMSendWhileDisconnectedError) {
     // reconnect logic
   } else if (err instanceof SMSendWhileNotReadyError) {
     // wait for 'connected' event
   }
 }

Checking connection errors during start():

 import { WebAPIHTTPError, WebAPIPlatformError, WebAPIRequestError } from '@slack/web-api';

 try {
   await client.start();
 } catch (err) {
   if (err instanceof WebAPIPlatformError) {
     console.log('Platform said:', err.data.error);
   } else if (err instanceof WebAPIHTTPError) {
     console.log('HTTP status:', err.statusCode);
   } else if (err instanceof WebAPIRequestError) {
     console.log('Network problem:', err.original.message);
   }
 }

Additionally, the SocketModeClient now uses proper instanceof checks when classifying errors from apps.connections.open to decide whether to auto-reconnect.

Requirements

• 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.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 826d039

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v8@187594b). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@          Coverage Diff          @@
##             v8    #2596   +/-   ##
=====================================
  Coverage      ?   87.39%           
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  Files         ?       62           
  Lines         ?    10083           
  Branches      ?      425           
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  Hits          ?     8812           
  Misses        ?     1248           
  Partials      ?       23           

Flag	Coverage Δ
cli-hooks	87.39% <100.00%> (?)
cli-test	87.39% <100.00%> (?)
logger	87.39% <100.00%> (?)
oauth	87.39% <100.00%> (?)
socket-mode	87.39% <100.00%> (?)
web-api	87.39% <100.00%> (?)
webhook	87.39% <100.00%> (?)

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