Add JsonElement.Parse

[stephentoub]

Closes #108930

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[Copilot]

Pull Request Overview

This PR introduces new JsonElement.Parse overloads for directly parsing JSON into a JsonElement, replaces existing JsonDocument.Parse(...).RootElement usage with the new API, updates tests to exercise and validate these overloads, and adds XML documentation for the new methods.

• Added JsonElement.Parse overloads accepting string, ReadOnlySpan<char>, and ReadOnlySpan<byte> with optional JsonDocumentOptions
• Refactored library and test code to use JsonElement.Parse in place of JsonDocument.Parse(...).RootElement
• Updated XML docs and added tests covering valid, invalid, and option-respecting parse scenarios

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
JsonElement.Parse.cs	Added public Parse overloads and XML documentation
JsonDocument.Parse.cs	Switched internal APIs to use ReadOnlySpan<char> instead of ReadOnlyMemory<char>
JsonNode.Parse.cs	Changed calls from ParseValue to new Parse overloads
JsonSerializer.Write.Element.cs	Updated serialization helpers to use JsonElement.Parse
Tests under System.Text.Json.Tests	Refactored tests to use JsonElement.Parse, added new tests for valid/invalid parsing and options

[gregsdennis]

@stephentoub could you describe a summary of the memory implications for these methods, please? If there's no JsonDocument, what owns the memory of the backing span? How is that managed?

I saw the ParseUnrented comment in the issue, and I figure that's related to memory, but I don't understand it.

[stephentoub]

@stephentoub could you describe a summary of the memory implications for these methods, please? If there's no JsonDocument, what owns the memory of the backing span? How is that managed?

It's the same as if you did JsonSerializer.Deserialize<JsonElement>(json). The resulting JsonElement is just wrapping a byte[] it owns.

[eiriktsarpalis]

Thanks