Add streaming-edit-file-tool feature flag (#49997)

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -8,7 +8,9 @@ use crate::{
 };
 use acp_thread::{MentionUri, UserMessageId};
 use action_log::ActionLog;
-use feature_flags::{FeatureFlagAppExt as _, SubagentsFeatureFlag};
+use feature_flags::{
+    FeatureFlagAppExt as _, StreamingEditFileToolFeatureFlag, SubagentsFeatureFlag,
+};
 
 use agent_client_protocol as acp;
 use agent_settings::{
@@ -2457,7 +2459,7 @@ impl Thread {
             }
         }
 
-        let use_streaming_edit_tool = false;
+        let use_streaming_edit_tool = cx.has_flag::<StreamingEditFileToolFeatureFlag>();
 
         let mut tools = self
             .tools

crates/feature_flags/src/flags.rs

@@ -56,3 +56,13 @@ impl FeatureFlag for DiffReviewFeatureFlag {
         false
     }
 }
+
+pub struct StreamingEditFileToolFeatureFlag;
+
+impl FeatureFlag for StreamingEditFileToolFeatureFlag {
+    const NAME: &'static str = "streaming-edit-file-tool";
+
+    fn enabled_for_staff() -> bool {
+        false
+    }
+}