This PR introduces a new `MultiBufferOffset` new type wrapping size. The
goal of this is to make it clear at the type level when we are
interacting with offsets of a multi buffer versus offsets of a language
/ text buffer. This improves readability of things quite a bit by making
it clear what kind of offsets one is working with while also reducing
accidental bugs by using the wrong kin of offset for the wrong API.
This PR also uncovered two minor bugs due to that.
Does not yet introduce the MultiBufferPoint equivalent, that is for a
follow up PR.
Release Notes:
- N/A *or* Added/Fixed/Improved ...
Previously we had `Context` and `ContextStore` in both `agent_ui` (used
to store context for the inline assistant) and `assistant_context` (used
for text threads) which is confusing.
This PR makes it so that the `assistant_context` concepts are now called
`TextThread*`, the crate was renamed to `assistant_text_thread`
Release Notes:
- N/A
