Extract ContextHistory to assistant_context_editor (#23437)

This PR extracts the `ContextHistory` to the `assistant_context_editor`
crate.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -50,7 +50,6 @@ menu.workspace = true
 multi_buffer.workspace = true
 parking_lot.workspace = true
 paths.workspace = true
-picker.workspace = true
 project.workspace = true
 prompt_library.workspace = true
 proto.workspace = true