Fix escaping bug in MutableJsonDocument

[annelo-msft]

Problem

In this AppConfig round-trip sample, if the highlighted line is commented out, the AppConfig service will respond to the request using DynamicData with a 400, where the same request using JsonDocument or JsonNode will succeed with a 200. This is because, prior to this PR, DynamicData was not handling escaping the same way JsonDocument does.

In the prior implementation, MutableJsonDocument.WriteTo() created a reader on the original JSON buffer, without doing any escaping, and passed reader.ValueSpan directly to writer.WriteStringValue(). Utf8JsonWriter will always escape string values (unless passed options that direct it otherwise), and JsonDocument deals with this by unescaping a string prior to calling WriteString().

Solution

Since all of the escaping logic in JsonDocument is held in internal types, the best we could do to duplicate the logic would be to copy those helper classes, or hand-write similar logic. This is clearly error prone. Instead, this PR moves the implementation of WriteTo() up a level of abstraction, so it can take advantage of the escaping logic in JsonDocument itself.

Implementation

This PR:

• Removes the Utf8JsonReader/Utf8JsonWriter implementation of MutableJsonDocument.WriteTo()
• Adds an internal JsonDocument/Utf8JsonWriter implementation of MutableJsonElement.WriteTo() and calls through to it from MutableJsonDocument.WriteTo()
• Updates tests for WriteTo() to compare the result of Mutable* WriteTo() methods to the result of JsonDocument.WriteTo() directly.

Contributes to #33856

[azure-sdk]

API change check

API changes are not detected in this pull request.

[heaths]

(Forgot to finish my review yesterday)