[nifragos]

Summary

With UseScopePerDispatch, 1.2.0 reused the in-flight dispatch scope for any nested Send/Publish. That included one made through a mediator resolved from a scope the handler had created on purpose to get its own DbContext, so the isolated work silently landed back on the outer DbContext.

Now only a mediator resolved from the in-flight scope (the ISender/IPublisher injected into a handler) joins it; any other mediator starts its own dispatch scope. Apps that don't call UseScopePerDispatch are unaffected.

• Mediator.Dispatch: reuse only when _provider is the active dispatch scope
• New test A_send_through_a_scope_the_handler_created_itself_is_not_folded_into_the_outer_dispatch
• XML docs, README (scope-per-dispatch section) and CHANGELOG updated; version 1.2.1

This branch is based on docs/upgrading-to-1.2 (#2) because it adds a 1.2.1 entry to the changelog introduced there. Merge #2 first, or merge this and close #2.

Test plan

• 60/60 tests pass on net10 locally; builds on net8/net9/net10 with warnings as errors
• The new test fails against the 1.2.0 reuse rule (Assert.NotSame – same instance) and passes with the fix
• Fabrica (the app that uses scope-per-dispatch) passes 176 application + 209 UI tests against a local 1.2.1 build, including nested-reuse and identity tests
• CI on net8/net9/net10