[2.7] Fix flake8 F401: remove unused imports in B1 pass-through tests

Remove three unused imports flagged by CI flake8:
  - test_pass_through_e2e.py: `from nvflare.fuel.utils import fobs`
  - test_pass_through_e2e.py: `from nvflare.fuel.utils.fobs import get_dot_handler`
  - test_pass_through.py: `_LAZY_BATCH_CTX_SUFFIX`

All 27 B1 tests still pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chesterxgchen

tests/unit_test/fuel/f3/streaming/test_pass_through_e2e.py

@@ -61,7 +61,6 @@
 import pytest
 
 from nvflare.fuel.f3.cellnet.cell import Cell
-from nvflare.fuel.utils import fobs
 from nvflare.fuel.utils.fobs import FOBSContextKey
 from nvflare.fuel.utils.fobs.decomposers.via_downloader import LazyDownloadRef, _RefKey
 from nvflare.fuel.utils.fobs.lobs import dump_to_bytes, load_from_bytes
@@ -226,7 +225,6 @@ def test_forwarded_payload_carries_original_server_ref(self, cells):
         """
         from io import BytesIO
 
-        from nvflare.fuel.utils.fobs import get_dot_handler
         from nvflare.fuel.utils.fobs.lobs import HEADER_LEN, MARKER_DATUM_TEXT, _Header
 
         server, _ = cells

tests/unit_test/fuel/utils/fobs/test_pass_through.py

@@ -35,7 +35,6 @@
 from nvflare.fuel.utils.fobs import FOBSContextKey
 from nvflare.fuel.utils.fobs.datum import Datum, DatumManager, DatumType
 from nvflare.fuel.utils.fobs.decomposers.via_downloader import (
-    _LAZY_BATCH_CTX_SUFFIX,
     EncKey,
     EncType,
     LazyDownloadRef,