fix(coding-agent): report edit access failures correctly (#3955)

* fix(coding-agent): report edit access failures correctly closes #3894

- classify edit and edit-preview access errors by errno
- add regressions for missing, permission, and fallback cases
- document the fix in the coding-agent changelog

* chore: get rid of CHANGELOG.md entry

* refactor(coding-agent): apply review suggestions - use single error msg

* refactor(coding-agent): clean up test cases

packages/coding-agent/src/core/tools/edit-diff.ts

@@ -412,8 +412,9 @@ export async function computeEditsDiff(
 		// Check if file exists and is readable
 		try {
 			await access(absolutePath, constants.R_OK);
-		} catch {
-			return { error: `File not found: ${path}` };
+		} catch (error: unknown) {
+			const errorMessage = error instanceof Error && "code" in error ? `Error code: ${error.code}` : String(error);
+			return { error: `Could not write file: ${path}. ${errorMessage}.` };
 		}
 
 		// Read the file

packages/coding-agent/src/core/tools/edit.ts

@@ -341,11 +341,13 @@ export function createEditToolDefinition(
 								// Check if file exists.
 								try {
 									await ops.access(absolutePath);
-								} catch {
+								} catch (error: unknown) {
+									const errorMessage =
+										error instanceof Error && "code" in error ? `Error code: ${error.code}` : String(error);
 									if (signal) {
 										signal.removeEventListener("abort", onAbort);
 									}
-									reject(new Error(`File not found: ${path}`));
+									reject(new Error(`Could not write file: ${path}. ${errorMessage}.`));
 									return;
 								}