Remove fallback to reflection converter resolution

[eiriktsarpalis]

Contributes to #71714 by removing fallback to the reflection converters for any resolver other than the default reflection-based resolver. More specifically this change:

• Makes contract resolvers the only source of truth when it comes to determining converters for types or properties.
• The reflection-based converter factories are only consulted by DefaultContractResolver (and JsonSerializerOptions.GetConverter whenever a resolver is left unspecified for backward compatibility purposes).
• Makes converter resolution for JsonPropertyInfo explicitly lazy unless the JsonTypeInfo for the declared type has already been cached.
• Changes the behavior of JsonSerializerOptions.GetConverter for options instances tied to JsonSerializerContext: the method will throw NotSupportedException for any type not explicitly supported by the context and will not query the reflection-based converter factories. This change is inline with the proposal in Remove implicit fallback to reflection-based serialization in System.Text.Json sourcegen #71714.
• Adds test coverage for recursive types containing Nullable<T> and F# optionals: types whose converters perform eager resolution for the converters of their element types.

Fix #71714.

[ghost]

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Issue Details

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Contributes to #71714 by removing fallback to the reflection converters for any resolver other than the default reflection-based resolver. More specifically this change:

• Makes contract resolvers the only source of truth when it comes to determining converters for types or properties.
• The reflection-based converter factories are only consulted by DefaultContractResolver (and JsonSerializerOptions.GetConverter whenever a resolver is left unspecified for backward compatibility purposes).
• Makes converter resolution for JsonPropertyInfo explicitly lazy unless the JsonTypeInfo for the declared type has already been cached.
• Changes the behavior of JsonSerializerOptions.GetConverter for options instances tied to JsonSerializerContext: the method will throw NotSupportedException for any type not explicitly supported by the context and will not query the reflection-based converter factories. This change is inline with the proposal in Remove implicit fallback to reflection-based serialization in System.Text.Json sourcegen #71714.
• Adds test coverage for recursive types containing Nullable<T> and F# optionals: types whose converters perform eager resolution for the converters of their element types.

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

[krwq]

LGTM with some comments