agent: Update old_text docs for StreamingEditFileTool (#50921)

In the old edit agent tool we encouraged the LLM to respond with whole
lines, which we did not do in the new edit file tool.

Release Notes:

- N/A

crates/agent/src/tools/streaming_edit_file_tool.rs

@@ -108,6 +108,11 @@ pub enum StreamingEditFileMode {
 pub struct Edit {
     /// The exact text to find in the file. This will be matched using fuzzy matching
     /// to handle minor differences in whitespace or formatting.
+    ///
+    /// Always include complete lines. Do not start or end mid-line.
+    /// Be minimal with replacements:
+    /// - For unique lines, include only those lines
+    /// - For non-unique lines, include enough context to identify them
     pub old_text: String,
     /// The text to replace it with
     pub new_text: String,