Walk struct properties that have something to validate

[tgharold]

Motivation

A struct in a property was never walked unless it was a collection. So [Range] on the Amount of a Money property, a positional record struct with [property: Range], an invalid object in a KeyValuePair<string, T> property, and a struct that wraps a class all passed validation. The same struct as an item of a list was validated. This closes those false passes for 3.0, which already carries breaking changes. Walking struct properties means reading getters the validator never read before, so the PR also denies the framework structs whose getters throw or wait.

Summary

• A struct property is walked when the struct is not a leaf type: it has a validation attribute, implements IValidatableObject, has a walked property such as KeyValuePair.Value, or is a collection of objects. A struct with nothing to validate, such as int, Guid or DateTime, is still skipped.
• A struct property that returns its own struct type, such as DateTime.Date, is not walked. Each read is a new copy that the walk can't match by reference, and without the rule DateTime would stop being a leaf type.
• The framework deny list adds GCHandle, INullable (all SqlTypes) and ValueTask<T>, matched by name because netstandard2.0 can't name it. A null SqlString or an unallocated GCHandle in an object property or a List<object> no longer throws. ValueTask<T>.Result is no longer read, even in an object property.
• Value tuple items are fields, so they are still not walked. The README says so.
• README gets a Structs section. The changelog gets a Fixed entry, a BREAKING Changed entry for the newly read getters and the deny list, and the new duplicate route in the struct known limit.

Probe of all 510 public runtime structs: under this rule 138 are newly walkable. 20 still throw on a default value: struct enumerators, async method builders, metadata-reader structs, DependentHandle, GCHandle<T> and PinnedGCHandle<T>. They are not model types and were already walked in an object property, so they are left off the deny list.

Test plan

• Specs first: 20 new specs fail against master, and the new guards pass there
• With the fix: 610 passed, 7 skipped on net8.0 and net10.0
• Test project builds for net481 with -p:IncludeNetFramework=true
• UpgradeCompatibilityTests still holds only shapes where 3.0 matches 2.2. The struct property moved to OddShapeTests.StructProperties
• Every new learn.microsoft.com link returns 200
• CI passes, including net481 on Windows