Closes ZED-4R9

Introduced in https://github.com/zed-industries/zed/pull/47768

The `insert_context_prefix` function was using byte offsets to check if
the prefix already exists at the cursor. This caused a panic with
multi-byte characters like emojis. Now uses character counts instead.

Release Notes:

- Fixed a crash in the Agent Panel when inserting context mentions with
emojis in the message editor.