Walk public instance properties only, not static ones

[tgharold]

Motivation

The validator walked public static properties as well as instance properties, because Type.GetProperties() with no arguments returns both. A static property holds data of the type, not of the model. Validator ignores it, and so does System.Text.Json. The effect was that a shared static object that was invalid failed every model of that type, with an error such as Shared.Name, and a static property that builds a new object on each read needed the equal-ancestor check or the depth limit to stop the walk.

Walking public instance properties only matches what a caller expects from DataAnnotations. It changes results, because a model that failed only because of a static property now passes, so it is marked BREAKING in the changelog.

This is part 3 of 3 small PRs, in this order: root collection (#86), nested class-level error path (#87), instance properties only (this one). This PR is stacked on #87 and targets its branch, so the diff shows only this change. Merge #86 and #87 first. After #87 merges, GitHub retargets this PR to master.

Summary

• Add GetWalkedProperties, which asks for public instance properties and applies the existing walk rules. The walk and the leaf-type check both use it, so they agree.
• A static property is no longer walked. Attributes on a static property never ran, and still do not.
• README and CHANGELOG: the README says that only public instance properties are walked, and the changelog has a BREAKING entry.

Test plan

• Full suite on net8.0 and net10.0: 588 passed, 7 skipped, 0 failed
• net481 test project builds with 0 warnings and 0 errors (-p:IncludeNetFramework=true)
• The new specs fail on the source of Give a nested error of a whole object the path of the object #87 (3 failures on net8.0), and pass with the fix. Commit order: specs, fix, docs.
• CI on Windows runs the net481 tests