fix: improve force_full test to set mtime before watermark

The test was failing in CI because restoring the original mtime still
resulted in a time newer than the watermark. Now we explicitly set
the mtime to 10 seconds before the watermark to ensure the incremental
scan won't detect the change, properly testing that force_full bypasses
the watermark optimization.

Issue #407

Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez

tests/sync/test_sync_service_incremental.py

@@ -177,19 +177,21 @@ async def test_force_full_bypasses_watermark_optimization(
     await sleep_past_watermark()
 
     # Modify a file WITHOUT updating mtime (simulates external tool like rclone)
-    # We do this by reading the current mtime, modifying the file, then restoring the mtime
+    # We set mtime to be BEFORE the watermark to ensure incremental scan won't detect it
     file_path = project_dir / "file1.md"
     original_stat = file_path.stat()
     await create_test_file(file_path, "# File 1\nModified by external tool")
-    # Restore original mtime to simulate external tool behavior
-    import os
 
-    os.utime(file_path, (original_stat.st_atime, original_stat.st_mtime))
+    # Set mtime to be before the watermark (use time from before first sync)
+    # This simulates rclone bisync which may preserve original timestamps
+    import os
+    old_time = initial_timestamp - 10  # 10 seconds before watermark
+    os.utime(file_path, (old_time, old_time))
 
-    # Normal incremental sync should NOT detect the change (mtime unchanged)
+    # Normal incremental sync should NOT detect the change (mtime before watermark)
     report = await sync_service.sync(project_dir)
     assert len(report.modified) == 0, (
-        "Incremental scan should not detect changes with unchanged mtime"
+        "Incremental scan should not detect changes with mtime older than watermark"
     )
 
     # Force full scan should detect the change via checksum comparison