Restore OnError routing for NPC-sourced WhenPropertyChanged emissions; bump main to v10 preview

[dwcullop]

Summary

PR reactivemarbles#1111 fixed a TOCTOU race in WhenPropertyChanged / WhenValueChanged, but a follow-up commit in that PR stopped routing downstream exceptions through OnError. Because these emissions originate from an INotifyPropertyChanged callback, an unhandled OnNext throw escaped raw out of the property setter that raised PropertyChanged. Setting a property could throw because some unrelated observer misbehaved.

This restores the error routing on both the shallow (x => x.Prop) and deep-chain (x => x.A.B.Prop) paths.

What changed

• SinglePropertySubscription.EmitCurrent and DeepChainSubscription.ProcessSignal again wrap the accessor read, chain walk, and the downstream OnNext in try/catch, routing any exception through OnError. A subscriber that supplied an error handler now observes the failure instead of it escaping into the setter.
• Per Jake's review point on WhenPropertyChanged: don't drop events fired during subscribe reactivemarbles/DynamicData#1111, we deliberately do not swallow a throw from OnError itself. An unhandled downstream error (or a rethrowing error handler) still propagates, matching Subject<T>.OnNext semantics. This is the middle ground between the original swallow-everything behavior and the raw-escape behavior that shipped.
• version.json: 9.5-preview -> 10.0-preview so main builds the next major preview.

Behavior

Scenario	Before this PR (current main)	After
Subscriber OnNext throws, has error handler	escapes raw to setter, handler bypassed	routed to OnError, setter safe
Subscriber OnNext throws, no error handler	escapes to setter	routed to OnError (no-op after detach), setter safe
Accessor throws, error handler rethrows	propagates to setter	propagates to setter (unchanged)

Tests

WhenPropertyChangedBehaviorFixture gains 4 tests (shallow + deep) covering the OnNext routing restore (RED on current main) and the non-swallow decision. Full Binding suite: 156 passed, 2 pre-existing skips.

Labels / breaking change

• manual-version-edit: required by pr-version-check.yml for a human edit to version.json.
• breaking-change: the major bump 9 -> 10, and the error-propagation behavior change, are both observable. Flagging so maintainers make the call.