[release/8.0-staging] Prefer most derived member in Configuration Binder source generator

[github-actions]

Backport of #101316 to release/8.0-staging

/cc @ericstj

Customer Impact

• Customer reported
• Found internally

When binding a type that uses new to hide a member and changes the type of that member, Configuration Binder will fail to bind. This is inconsistent with the reflection binder, which binds all members.

Regression

• Yes
• No

Not a regression, but a blocker when using the source-generator.

Testing

New automated tests added, sharing bits with partner team to validate scenario is unblocked.

Risk

Low. Previously the source generator would traverse the type hierarchy down to System.Object and use the lowest/base member definition. Now we’ll choose the highest/most derived.

It could mean that there were cases that produced compiler failures before that now become warnings. For example - a type hides a member to make it read-only. Previously the source generator would try to set that member and result in an error. Now the member will be ignored, just like any other read-only member. This new behavior is more desirable and consistent with the behavior of the source generator.

There's still a gap between the source-generator and the reflection binder. The reflection binder would set all members in the type hierarchy with the same configuration data - even if the member was hidden. We're not trying to replicate this behavior and have not heard customer feedback asking for that behavior in the source generator.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-extensions-configuration
See info in area-owners.md if you want to be subscribed.

[tarekgh]

@ericstj I assume you'll fill the template and add the package authoring if needed. Thanks!

[eerhardt]

I took a private build of this and used it in the project I was hitting #101267 and #101273. This change allows the generated code to build successfully. It should unblock my scenario.

[ericstj]

Approved in mail.