fix(diagnostics_channel): make channel.hasSubscribers a getter property

[prydt]

Node.js' Channel.hasSubscribers and TracingChannel.hasSubscribers as being boolean getter properties, not functions. In workerd, they are currently implemented as functions which means that if anyone tries to do something like

if(channel.hasSubscribers) // always truthy

https://nodejs.org/dist/latest-v20.x/docs/api/diagnostics_channel.html#channelhassubscribers

This should make workerd's behavior match Node.js for diagnostics_channel. I've added a couple tests here but I'm open to suggestions for ways to improve this if needed.

Here is a worker demonstrating the issue: https://diagnostics-channel-hassubscribers-bug.pranoydll.workers.dev/

[guybedford]

Thanks for posting the fix.

[jasnell]

Fortunately, this is going to need a compat flag in order to land

[prydt]

Fortunately, this is going to need a compact flag in order to land

That makes sense. I'll add a compat flag and report back. Thanks!

[prydt]

Added the compat flag and opened a docs PR. Set a placeholder compat date of 2026-05-01, but that might be too early so feel free to suggest a date you think is more reasonable.

(The add-compat-flag SKILL was extremely useful!)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.57%. Comparing base (d1e13ad) to head (975039b).
⚠️ Report is 78 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6601      +/-   ##
==========================================
- Coverage   70.85%   66.57%   -4.29%     
==========================================
  Files         438      405      -33     
  Lines      123644   117527    -6117     
  Branches    19455    19381      -74     
==========================================
- Hits        87608    78239    -9369     
- Misses      24509    27716    +3207     
- Partials    11527    11572      +45     

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[jasnell]

Manual run of the internal build job is clear. Doc PR opened. Merging.