Hi Zed team! 👋
As a fan of Zed Editor who's excited to contribute, I noticed a small
optimization opportunity in the JSON merging utilities.

<b>Changes:</b>
Removed redundant key.clone() calls in insert() operations within 2
functions:
1. merge_json_value_into
2. merge_non_null_json_value_into

<b>Why:</b>
Since we're already moving ownership of `source_object` 's contents and
key is no longer used further, we could directly pass `key` to `insert`
without cloning.
Eliminates redundant allocations, improving performance slightly for
JSON-heavy operations.

<b>Testing:</b>
I have tested this locally and all existing tests passed. The change
preserves behavior while removing redundancy.

Love using Zed and happy to contribute! Let me know if any adjustments
are needed!

Release Notes:

- N/A