diagnostics_channel: return a disposable from subscribe

[araujogui]

Make subscribe return a Disposable so a subscription can be scoped with
using instead of pairing each call with a matching unsubscribe.

const channel = diagnostics_channel.channel('my-channel');

{
  using subscription = channel.subscribe((message, name) => {});
  channel.hasSubscribers; // true
}
channel.hasSubscribers; // false

Applies to diagnostics_channel.subscribe(), channel.subscribe(),
tracingChannel.subscribe(), and boundedChannel.subscribe(). The disposable
removes only the handlers that call registered, and disposing is idempotent.
Existing callers are unaffected — the return value was previously undefined.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (cf30b2e) to head (27df399).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65383      +/-   ##
==========================================
+ Coverage   90.11%   90.13%   +0.01%     
==========================================
  Files         752      752              
  Lines      251861   251919      +58     
  Branches    47365    47373       +8     
==========================================
+ Hits       226955   227055     +100     
+ Misses      16238    16186      -52     
- Partials     8668     8678      +10     

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	97.66% <100.00%> (+0.35%)	⬆️

... and 36 files with indirect coverage changes

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