## Context

Closes #46698

This PR fixes a bug where the `cs` (change surrounds) operator fails on
symmetric delimiters like quotes.

In a sequence like `c s ' "`, Zed previously performed two independent
searches. For a line like `I'm 'goˇod'`, the first step (`cs'`)
correctly moves the cursor to the second quote: `I'm ˇ'good'`. However,
when the replacement character `"` is pressed, `change_surrounds` would
perform another scan from the new cursor position. Because quotes are
symmetric, this second search would incorrectly match `'m '` as the
target pair, leading to a broken result like `I"m "good'`.

I've refactored the workflow to ensure the search happens only once.
`prepare_and_move_to_valid_bracket_pair` now computes and stores the
`Anchor` positions of the detected pair directly into the operator
state. `change_surrounds` then simply reuses these anchors instead of
re-executing the search. This ensures correctness for quotes while
remaining consistent with Vim/Neovim cursor behavior. While this
slightly increases coupling between these two functions, it is an
intentional trade-off since they exclusively serve the `cs` operation.

## How to Review

The main changes are in `crates/vim/src/surrounds.rs`. I renamed
`check_and_move_to_valid_bracket_pair` to
`prepare_and_move_to_valid_bracket_pair` and updated it to return the
detected bracket anchors. In `change_surrounds`, I removed the redundant
search logic and updated it to perform the replacement using the
provided anchors. You can also see the updated
`Operator::ChangeSurrounds` enum variant in `crates/vim/src/state.rs`
which now carries the anchor data.


## Self-Review Checklist

<!-- Check before requesting review: -->
- [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

Release Notes:

- Fixed an issue where the `cs` Vim operator incorrectly identified
symmetric quotes in certain contexts.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>