Ensure that JsonSerializerOptions.CacheContext is a singleton.

[eiriktsarpalis]

Fixes a race condition wherein the JsonSerializerOptions.CacheContext property could be returning multiple instances, which can occasionally make this assertion fail:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs

Line 672 in 0ad3723

Debug.Assert(Monitor.IsEntered(Options.CacheContext), "Configure called directly, use EnsureConfigured which synchronizes access to this method");

Fix #88102.

[ghost]

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Issue Details

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Fixes a race condition wherein the JsonSerializerOptions.CacheContext property could be returning multiple instances, which can occasionally make this assertion fail:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs

Line 672 in 0ad3723

Debug.Assert(Monitor.IsEntered(Options.CacheContext), "Configure called directly, use EnsureConfigured which synchronizes access to this method");

Fix #88102.

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