Allow to regenerate a summary of the assistant context (#14964)

Both manual and LLM-through ways are supported:


https://github.com/user-attachments/assets/afb0d2b3-9a9b-4f78-a909-1e663e686323


Release Notes:

- Improved assistant panel summarization usability

crates/assistant/Cargo.toml

@@ -26,7 +26,6 @@ anyhow.workspace = true
 assets.workspace = true
 assistant_slash_command.workspace = true
 async-watch.workspace = true
-breadcrumbs.workspace = true
 cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true