Make a number of improvements to the System.Text.Json source generator

[eiriktsarpalis]

Makes a number of improvements to the System.Text.Json source generator:

1. Ensures that the source generated JsonTypeInfo<T> singleton instances are populated via the corresponding JsonSerializerOptions cache and are thus preconfigured before they are surfaced to users. The generated metadata properties now look as follows:

    internal partial class ClassWithStringValuesContext
    {
        private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String>? _String;
            
        /// <summary>
        /// Defines the source generated JSON serialization contract metadata for a given type.
        /// </summary>
        public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String> String
        {
            get => _String ??= (global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String>)Options.GetTypeInfo(typeof(global::System.String));
        }
    }

2. Fixes indentation & whitespace issues with generated source code, particularly with contexts not in namespaces.
3. Fix JSON source generator emits CS0612 warnings if serialized types use [Obsolete] #76829.

I've split the relevant changes into separate commits for easier reviewing.

[ghost]

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Issue Details

---

Makes a number of improvements to the System.Text.Json source generator:

1. Ensures that the source generated JsonTypeInfo<T> singleton instances are populated via the corresponding JsonSerializerOptions cache and are thus preconfigured before they are surfaced to users. The generated metadata properties now look as follows:

    internal partial class ClassWithStringValuesContext
    {
        private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String>? _String;
            
        /// <summary>
        /// Defines the source generated JSON serialization contract metadata for a given type.
        /// </summary>
        public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String> String
        {
            get => _String ??= (global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.String>)Options.GetTypeInfo(typeof(global::System.String));
        }
    }

2. Fixes indentation & whitespace issues with generated source code, particularly with contexts not in namespaces.
3. Fix JSON source generator emits CS0612 warnings if serialized types use [Obsolete] #76829.

I've split the relevant changes into separate commits for easier reviewing.

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