agent: Snapshot context in user message instead of recreating it (#27967)

This makes context essentially work the same way as `read-file`,
increasing the likelihood of cache hits.

Just like with `read-file`, we'll notify the model when the user makes
an edit to one of the tracked files. In the future, we want to send a
diff instead of just a list of files, but that's an orthogonal change.


Release Notes:
- agent: Improved caching of files in context

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/agent/src/thread_store.rs

@@ -374,6 +374,8 @@ pub struct SerializedMessage {
     pub tool_uses: Vec<SerializedToolUse>,
     #[serde(default)]
     pub tool_results: Vec<SerializedToolResult>,
+    #[serde(default)]
+    pub context: String,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -441,6 +443,7 @@ impl LegacySerializedMessage {
             segments: vec![SerializedMessageSegment::Text { text: self.text }],
             tool_uses: self.tool_uses,
             tool_results: self.tool_results,
+            context: String::new(),
         }
     }
 }