Encourage use of start_line in multi-file diff to match legacy diff

src/core/diff/strategies/multi-file-search-replace.ts

@@ -107,12 +107,12 @@ Parameters:
   - path: (required) The path of the file to modify (relative to the current workspace directory ${args.cwd})
   - diff: (required) One or more diff elements containing:
     - content: (required) The search/replace block defining the changes.
-    - start_line: (optional) The line number of original content where the search block starts.
+    - start_line: (required) The line number of original content where the search block starts.
 
 Diff format:
 \`\`\`
 <<<<<<< SEARCH
-:start_line: (optional) The line number of original content where the search block starts.
+:start_line: (required) The line number of original content where the search block starts.
 -------
 [exact content to find including whitespace]
 =======
@@ -294,7 +294,7 @@ Each file requires its own path, start_line, and diff elements.
 				"\n" +
 				"CORRECT FORMAT:\n\n" +
 				"<<<<<<< SEARCH\n" +
-				":start_line: (optional) The line number of original content where the search block starts.\n" +
+				":start_line: (required) The line number of original content where the search block starts.\n" +
 				"-------\n" +
 				"[exact content to find including whitespace]\n" +
 				"=======\n" +

src/core/tools/multiApplyDiffTool.ts

@@ -153,7 +153,7 @@ Expected structure:
     <path>relative/path/to/file.ext</path>
     <diff>
       <content>diff content here</content>
-      <start_line>optional line number</start_line>
+      <start_line>line number</start_line>
     </diff>
   </file>
 </args>