assistant2: Summarize threads in context and continue long ones (#27851)

We'll now prompt the user to start a new thread when the active one gets
too long.

<img width=500
src="https://github.com/user-attachments/assets/91445bc0-3e81-422f-aa4a-b8f0741f9d9a"></img>


When they click "Start New Thread", will create a new one with the
previous one added as context.

<img width=500
src="https://github.com/user-attachments/assets/c3b4223f-5bdd-4ba4-956f-5a5880d5e2c3"></img>

Instead of including the full thread text, we'll now add summarized
versions of threads to the context, allowing you to continue the
conversation even if it was near the token limit.

- Thread summaries are cached and persisted. 
- A cached summary is invalidated if the thread is continued.
- We start generating the thread summary as soon as it's selected from
the picker. Most times, the summary will be ready by the time the user
sends the message.
- If the summary isn't ready by the time a message is sent, the user
message will be displayed in the thread immediately, and a "Summarizing
context..." indicator will appear. After the summaries are ready, we'll
start generating the response and show the usual "Generating..."
indicator.

Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/assistant2/src/assistant.rs

@@ -33,6 +33,7 @@ use prompt_store::PromptBuilder;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use settings::Settings as _;
+use thread::ThreadId;
 
 pub use crate::active_thread::ActiveThread;
 use crate::assistant_configuration::{AddContextServerModal, ManageProfilesModal};
@@ -45,7 +46,6 @@ pub use assistant_diff::{AssistantDiff, AssistantDiffToolbar};
 actions!(
     agent,
     [
-        NewThread,
         NewPromptEditor,
         ToggleContextPicker,
         ToggleProfileSelector,
@@ -73,6 +73,12 @@ actions!(
     ]
 );
 
+#[derive(Default, Clone, PartialEq, Deserialize, JsonSchema)]
+pub struct NewThread {
+    #[serde(default)]
+    from_thread_id: Option<ThreadId>,
+}
+
 #[derive(PartialEq, Clone, Default, Debug, Deserialize, JsonSchema)]
 pub struct ManageProfiles {
     #[serde(default)]
@@ -87,7 +93,7 @@ impl ManageProfiles {
     }
 }
 
-impl_actions!(agent, [ManageProfiles]);
+impl_actions!(agent, [NewThread, ManageProfiles]);
 
 const NAMESPACE: &str = "agent";