fix(error-handling): _handle_file_edit() now respects encoding parameter

- Fixed _handle_file_edit() to use encoding=inputs.get("encoding", "utf-8") for both read_text() and write_text() calls, ensuring encoding is explicit and not dependent on the platform default.
- Added encoding field to the FILE_EDIT_SPEC schema to surface encoding choices to callers.

ISSUES CLOSED: #7559

features/steps/tool_builtins_steps.py

@@ -62,6 +62,15 @@ def step_given_file_with_content(context: Any, name: str, content: str) -> None:
     path.write_text(content)
 
 
+@given('an encoded file "{name}" with encoding "{encoding}" and content "{content}"')
+def step_given_file_with_content_encoding(
+    context: Any, name: str, encoding: str, content: str
+) -> None:
+    path = Path(context.sandbox_dir) / name
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(content, encoding=encoding)
+
+
 @given('a file "{name}" with lines "{csv_lines}"')
 def step_given_file_with_lines(context: Any, name: str, csv_lines: str) -> None:
     lines = csv_lines.split(",")
@@ -149,6 +158,39 @@ def step_when_file_edit(context: Any, old: str, new: str) -> None:
     )
 
 
+@when(
+    'I edit file "edit-enc.txt" replacing "{old}" with "{new}" specifying encoding "{encoding}"'
+)
+def step_when_file_edit_with_encoding(
+    context: Any, old: str, new: str, encoding: str
+) -> None:
+    _run_tool(
+        context,
+        "builtin/file-edit",
+        {
+            "path": "edit-enc.txt",
+            "old_text": old,
+            "new_text": new,
+            "encoding": encoding,
+        },
+    )
+
+
+def step_when_file_edit_with_encoding(
+    context: Any, old: str, new: str, encoding: str
+) -> None:
+    _run_tool(
+        context,
+        "builtin/file-edit",
+        {
+            "path": "edit-enc.txt",
+            "old_text": old,
+            "new_text": new,
+            "encoding": encoding,
+        },
+    )
+
+
 @when('I execute the "builtin/file-edit" tool replacing all "{old}" with "{new}"')
 def step_when_file_edit_all(context: Any, old: str, new: str) -> None:
     _run_tool(