edit_file: Let agent specify locations of edit chunks (#32628)

These changes help the agent edit files when `<old_text>` matches more
than one location.

First, the agent can specify an optional `<old_text line=XX>` parameter.
When this is provided and multiple matches exist, we use this hint to
identify the best match.

Second, when there is ambiguity in matches, we now return the agent a
more helpful message listing the line numbers of all possible matches.

Together, these changes should reduce the number of misplaced edits and
agent confusion.

I have ensured the LLM Worker works with these prompt changes.


Release Notes:

- Agent: Improved locating edits

crates/assistant_tools/src/edit_agent/evals.rs

@@ -438,14 +438,21 @@ fn eval_disable_cursor_blinking() {
 #[test]
 #[cfg_attr(not(feature = "eval"), ignore)]
 fn eval_from_pixels_constructor() {
-    // Results for 2025-05-22
+    // Results for 2025-06-13
+    //
+    // The outcome of this evaluation depends heavily on the LINE_HINT_TOLERANCE
+    // value. Higher values improve the pass rate but may sometimes cause
+    // edits to be misapplied. In the context of this eval, this means
+    // the agent might add from_pixels tests in incorrect locations
+    // (e.g., at the beginning of the file), yet the evaluation may still
+    // rate it highly.
     //
     //  Model                          | Pass rate
     // ============================================
     //
-    //  claude-3.7-sonnet              |
-    //  gemini-2.5-pro-preview-03-25   |  0.94
-    //  gemini-2.5-flash-preview-04-17 |
+    //  claude-4.0-sonnet              |  0.99
+    //  claude-3.7-sonnet              |  0.88
+    //  gemini-2.5-pro-preview-03-25   |  0.96
     //  gpt-4.1                        |
     let input_file_path = "root/canvas.rs";
     let input_file_content = include_str!("evals/fixtures/from_pixels_constructor/before.rs");