TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 4: refactor(gui_2): migrate L3740 render_ast_inspector_modal file_content to Result[T] (Phase 4)

Adds _render_ast_inspector_file_content_result(app, f_path) -> Result[str | None]
helper that wraps the mcp_client.read_file try/except in render_ast_inspector_modal.
On success, returns the file content string. On failure, returns Result(data=None,
errors=[ErrorInfo]). The legacy wrapper handles the side effects (sets
app._cached_ast_file_lines + app.text_viewer_content) and drains errors to
app._last_request_errors (per FR-BC-3 modal pattern; data plane attribute).

Audit: BROAD_CATCH count 15 -> 14, COMPLIANT count 22 -> 23. Migration
target count drops by 1. All 3 Phase 4 sites migrated. Tests: 2/2 pass.

tests/test_gui_2_result.py

@@ -698,3 +698,42 @@ def test_phase_4_l3718_render_ast_inspector_outline_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._render_ast_inspector_outline_result"
  assert "Error fetching outline" in result.data
+
+
+def test_phase_4_l3740_render_ast_inspector_file_content_result_success():
+ """
+ L3740 _render_ast_inspector_file_content_result returns Result.ok=True on success.
+
+ The helper wraps the mcp_client.read_file try/except in
+ render_ast_inspector_modal. On success, returns Result(data=content)
+ where content is the file content string. The caller sets
+ app._cached_ast_file_lines and app.text_viewer_content from result.data.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ with patch.object(gui_2.mcp_client, "read_file", return_value="line1\nline2\nline3") as _rf:
+  result = gui_2._render_ast_inspector_file_content_result(app, "/proj/foo/src/bar.py")
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data == "line1\nline2\nline3"
+
+
+def test_phase_4_l3740_render_ast_inspector_file_content_result_failure():
+ """
+ L3740 _render_ast_inspector_file_content_result returns Result.ok=False on failure.
+
+ When mcp_client.read_file raises, the helper returns Result(data=None,
+ errors=[ErrorInfo]). The legacy wrapper drains to app._last_request_errors
+ (per FR-BC-3 modal pattern) and sets app._cached_ast_file_lines to the
+ fallback error message.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ with patch.object(gui_2.mcp_client, "read_file", side_effect=RuntimeError("read failed")):
+  result = gui_2._render_ast_inspector_file_content_result(app, "/proj/foo/src/bar.py")
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._render_ast_inspector_file_content_result"
+ assert result.data is None