## Context

`update_settings_file` deletes unrelated settings when the settings file
contains deprecated keys that need migration. For example, changing the
model from the Agent Panel overwrites the entire `agent` block instead
of just updating `default_model`.

The root cause is that `edits_for_update` used
`parse_json_with_comments` (strict parser), which returns `Err` on
deprecated/unknown fields. The error is swallowed by `log_err()`,
falling back to `Default::default()` (empty settings). The diff then
sees everything as new and replaces the entire block.

The fix switches to `parse_json` (the fallible/lenient parser), which
returns `Some(parsed_value)` even when deprecated fields are present -
the same pattern already used by `parse_and_migrate_zed_settings`.

## Fixes #41344

## How to Review

Single-file change in `settings_store.rs`, focus on `edits_for_update` .
Compare with `parse_and_migrate_zed_settings` (line 702) which already
uses the same `parse_json` approach.

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Video
[Screencast from 2026-03-21
00-36-12.webm](https://github.com/user-attachments/assets/31bd584a-2674-4c91-bdb2-69ed8fa35e88)


### Note : Reopens previous work from closed PR #52081 (fork was
deleted)

Release Notes:

- Fixed settings being overwritten when updating a single setting via UI
while the settings file contains deprecated keys.