fix(serialization): Replace redundant round-trip in JsonSerializer.Parse with Clone

[SindriFr]

Summary of the changes (Less than 80 chars)

• Improve performance of JsonSerializer.Parse
• Avoid buffer overflow bug in System.Text.Json WriteTo for complex nested JSON objects

[CLAassistant]

All committers have signed the CLA.

[SindriFr]

The JsonSerializer.Parse method was performing an unnecessary round-trip serialization (JsonElement -> Utf8JsonWriter -> Utf8JsonReader -> JsonElement).

More critically, the WriteTo() operation triggered a buffer overflow bug in System.Text.Json's WriteComplexElement when processing complex nested JSON objects larger than Utf8JsonWriter's internal buffer of 4KB, leading to memory corruption and undefined behaviour.

Replace the round-trip with JsonElement.Clone() which:

• Preserves identical semantics
• Eliminates the buffer overflow risk entirely
• Improves performance by avoiding unnecessary serialization
• Maintains proper memory management for the cloned element

Sorry I didn't set up a bug report before opening the pull request, but reproducing this bug outside my application has been a challenge.
I hope the performance improvement and simplification is enough to warrant a merge.

[michaelstaib]

@SindriFr thanks for the PR!