Fix InvalidCastException when casting a polymorphic converter

[eiriktsarpalis]

Has been largely fixed thanks to the infrastructural changes in #70435 and #72789. This adds a fix for the case of dictionary converters that was missed out and includes a regression test for the original bug report.

Fix#46522.

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Issue Details

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Has been largely fixed thanks to the infrastructural changes in #70435 and #72789. This adds a fix for the case of dictionary converters that was missed out and includes a regression test for the original bug report.

Fix #46522.

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

[eiriktsarpalis]

I've expanded the scope of this PR somewhat, incorporating equivalent fixes & testing for the serialization case. The PR also includes a few additional changes, tidying up logic around the casting converter and ensuring we never layer multiple casting converters on top of each other.