Fix duplicate connection/logs in Webdev

[jyameo]

Fix duplicate app logs on page refresh in Webdev
Fixes: #2604

Problem
Each page refresh created duplicate stdout listeners for the same app, causing exponential log duplication (1 log → 2 logs → 3 logs, etc.).

Root Cause
app_domain.dart created new VM service stdout listeners on every dwds.connectedApps event without checking if listeners already existed for the same appId.

Solution
Added simple tracking to prevent duplicate listeners:

• Track active appIds in a Set _activeListeners
• Only create stdout listeners if not already listening for that appId
• Clean up tracking when connections end

Changes
app_domain.dart: Added listener deduplication with minimal code changes

Result

• Single log event per refresh (no more duplicates)
• All debugging functionality preserved
• No breaking changes

[auto-submit]

autosubmit label was removed for dart-lang/webdev/2635, because - The status or check suite publish / validate has failed. Please fix the issues identified (or deflake) before re-applying this label.

[bkonyi]

Just a couple minor comments, but LGTM!

[jyameo]

@bkonyi I made the following changes:

• use _appStates.containsKey(appId) to check for existing connections.
• I removed the _activeListeners set entirely since the check is happening at the AppStates level.
• Also removed the unnecessary vmService.streamCancel('Stdout');
• I wasn't sure about server.buildResults so I left it as is for now.

Let me know what you think and If I missed something here. Thanks :)