style(ui): fix ruff lint (unused imports, import order) in Task 1.6 tests

Repo-wide 'ruff check .' (CI 'Python UI' lint step) flagged unused imports
(MagicMock, Investigation) and import ordering that per-file agent checks missed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ethompsy

ui/beeper_ui/routes/react_shell.py

@@ -21,7 +21,6 @@
 it works in both environments without environment variables.
 """
 
-import os
 from pathlib import Path
 
 from flask import Blueprint, send_file, send_from_directory

ui/tests/test_api_v1_investigations.py

@@ -21,7 +21,7 @@
 from __future__ import annotations
 
 import json
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 
 import pytest
 import respx
@@ -32,12 +32,10 @@
 from beeper_ui.app import create_app
 from beeper_ui.config import TestingConfig
 from beeper_ui.services.investigation_service import (
-    Investigation,
     InvestigationDetail,
     InvestigationServiceError,
 )
 
-
 # ---------------------------------------------------------------------------
 # Fixtures
 # ---------------------------------------------------------------------------