Refactor root-level serialization logic and polymorphic value handling.

[eiriktsarpalis]

Refactors the root-level serialization/deserialization routines so that

1. Both JsonSerializerOptions and JsonTypeInfo<T> overloads in JsonSerializer call into common helper methods, ensuring that metadata is being handled consistently regardless of invoked overload.
2. Polymorphic handling of root-level object values is moved to the converter layer. Fixes System.Text.Json instances of polymorphic types emit type discriminator when serialized as object types #72187 and System.Text.Json uses inconsistent polymorphism semantics with custom object converters #72681.

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Issue Details

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Refactors the root-level serialization/deserialization routines so that

1. Both JsonSerializerOptions and JsonTypeInfo<T> overloads in JsonSerializer call into common helper methods, ensuring that metadata is being handled consistently regardless of invoked overload.
2. Polymorphic handling of root-level object values is moved to the converter layer. Fixes System.Text.Json instances of polymorphic types emit type discriminator when serialized as object types #72187 and System.Text.Json uses inconsistent polymorphism semantics with custom object converters #72681.

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

[eiriktsarpalis]

It must be noted that this change introduces a slight performance regression, specifically when it comes to serializing small root-level object values:

System.Text.Json.Serialization.Tests.WriteJson<Int32>.SerializeObjectProperty

Method	Job	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	MannWhitney(3%)	RatioSD	Gen 0	Allocated	Alloc Ratio
SerializeObjectProperty	Job-LSKJVH	main	235.7 ns	5.44 ns	5.82 ns	236.1 ns	226.0 ns	246.8 ns	1.00	Base	0.00	0.0254	264 B	1.00
SerializeObjectProperty	Job-KLBYHL	PR	249.3 ns	8.82 ns	10.16 ns	249.8 ns	231.8 ns	270.1 ns	1.06	Slower	0.04	0.0260	264 B	1.00

Fundamentally, it is caused by this change removing the hardcoded polymorphic serialization for root-level object values. Instead of directly resolving the metadata for the runtime type, the metadata for object is consulted before proceeding with polymorphic serialization. As such, in the worst-case scenario, two lookups need to be made against the metadata cache instead of just one.

I've been able to recover some of the performance losses by employing a secondary LRU cache specifically used for root-level polymorphic types. I think we should merge this PR and accept the performance regression, as it is essential for getting #72187 and #72681 fixed. The performance regression only impacts small, root-level polymorphic values and does not affect the statically typed APIs or the source generator which are more performance-minded.

cc @jeffhandley @steveharter @ericstj

[krwq]

If incorrect code or code wrote under wrong assumptions is faster then there is no perf regression to me. We should baseline from correct code. If there are any quick wins to gain the perf back then even better

[eiriktsarpalis]

I made a few tweaks to the caching strategy in 97a8b7a, caching the resolved JsonTypeInfo<object> instead of using a secondary LRU cache for polymorphic types, which has contributed to even more performance gains:

Method	Job	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	MannWhitney(3%)	RatioSD	Gen 0	Allocated	Alloc Ratio
SerializeObjectProperty	Job-LZOHNV	main	257.1 ns	10.66 ns	11.85 ns	259.2 ns	238.0 ns	281.6 ns	1.00	Base	0.00	0.0261	264 B	1.00
SerializeObjectProperty	Job-RKZQDC	PR	268.1 ns	7.07 ns	7.85 ns	268.3 ns	247.5 ns	279.9 ns	1.05	Same	0.06	0.0258	264 B	1.00