fix(observables): deliver property changes on the raising thread

[glennawatson]

What kind of change does this PR introduce?

Bug fix.

What is the new behavior?

WhenChanged, WhenAnyValue and WhenAny deliver each property change on the thread that set the property.

• Only the binding operators (Bind, BindTwoWay, OneWayBind, BindOneWay, BindTo, BindCommand) move work onto a thread, because they connect a view to a view model.
• The observation operators are for general use, so the caller supplies the threading arrangement.
• A setter on one thread waits for at most the delivery already running on another thread.
• A change raised from inside a running delivery is delivered after the subscriber returns.

What is the current behavior?

When two threads set the same property at once, one thread delivers the other thread's changes.

• An observation subscriber can run on a thread other than the one that set the property.
• A thread already delivering a change is held for as long as the other thread keeps setting the property.

What might this PR break?

None

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

This fixes an unintended threading change that shipped in 7.0.0 and 7.0.1 through #100.

• Only the binding operators are meant to change the thread work runs on; fix(binding): settle two-way bindings and drop needless locks #100 also changed the thread WhenChanged, WhenAnyValue and WhenAny deliver on.
• 7.0.0 and 7.0.1 are withdrawn in favour of the release that carries this fix.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.73%. Comparing base (e200cb3) to head (04c9743).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #105   +/-   ##
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  Coverage   99.73%   99.73%           
=======================================
  Files         289      289           
  Lines       11533    11537    +4     
  Branches     1259     1259           
=======================================
+ Hits        11502    11506    +4     
  Misses         18       18           
  Partials       13       13           

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