Rename assistant_context crate to assistant_text_thread (#41024)

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

crates/paths/src/paths.rs

@@ -288,7 +288,7 @@ pub fn snippets_dir() -> &'static PathBuf {
 /// Returns the path to the contexts directory.
 ///
 /// This is where the saved contexts from the Assistant are stored.
-pub fn contexts_dir() -> &'static PathBuf {
+pub fn text_threads_dir() -> &'static PathBuf {
     static CONTEXTS_DIR: OnceLock<PathBuf> = OnceLock::new();
     CONTEXTS_DIR.get_or_init(|| {
         if cfg!(target_os = "macos") {