Fix a number of issues related to boxed value serialization

[eiriktsarpalis]

This PR:

• Removes unspeakable type support form serialization overloads that accept an explicit Type parameter. Unspeakable type support now only works for values that are serialized as object types. Fix Add source gen support for unspeakable types #82457.
• Update the type hierarchies feature so that polymorphism metadata is honored when serializing boxed values. This is done using a nearest polymorphic ancestor traversal algorithm similar to how unspeakable type support is implemented. Fix System.Text.Json Polymorphic Type Resolving Issue #77532.
• Adds support for boxed root value serialization in source generators that don't supply metadata for object. Fix JSON Serialize as object doesn't work with source generation by default #84704.

I've split up the PR into three separate commits for easier reviewing.

[ghost]

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Issue Details

---

This PR:

• Removes unspeakable type support form serialization overloads that accept an explicit Type parameter. Unspeakable type support now only works for values that are serialized as object types. Fix Add source gen support for unspeakable types #82457.
• Update the type hierarchies feature so that polymorphism metadata is honored when serializing boxed values. This is done using a nearest polymorphic ancestor traversal algorithm similar to how unspeakable type support is implemented. Fix System.Text.Json Polymorphic Type Resolving Issue #77532.
• Adds support for boxed root value serialization in source generators that don't supply metadata for object. Fix JSON Serialize as object doesn't work with source generation by default #84704.

I've split up the PR into three separate commits for easier reviewing.

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eerhardt]

The 1st and 3rd commits LGTM. The 2nd was more complex and I didn't completely understand all of it. Hopefully someone on the STJ team can give that commit a review.