Fixed exceptions when closing AppendingTiffWriter

[radarhere]

Two exceptions are being raised in testing - https://github.com/python-pillow/Pillow/actions/runs/13051364088/job/36412180284#step:11:4911

Tests/test_file_tiff.py::TestFileTiff::test_appending_tiff_writer_writelong
  /Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <PIL.TiffImagePlugin.AppendingTiffWriter object at 0x114c785e0>
  
  Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/PIL/TiffImagePlugin.py", line 2212, in close
      self.finalize()
      ~~~~~~~~~~~~~^^
    File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/PIL/TiffImagePlugin.py", line 2082, in finalize
      raise RuntimeError(msg)
  RuntimeError: IIMM of new page doesn't match IIMM of first page
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

Tests/test_file_tiff.py::TestFileTiff::test_appending_tiff_writer_rewritelastshorttolong
  /Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <PIL.TiffImagePlugin.AppendingTiffWriter object at 0x114c7ade0>
  
  Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/PIL/TiffImagePlugin.py", line 2212, in close
      self.finalize()
      ~~~~~~~~~~~~~^^
    File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages/PIL/TiffImagePlugin.py", line 2082, in finalize
      raise RuntimeError(msg)
  RuntimeError: IIMM of new page doesn't match IIMM of first page
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

These exceptions are occurred when AppendingTiffWriter is being closed at the end of the test method.

Pillow/Tests/test_file_tiff.py

Lines 779 to 791 in e4f2a4a

	def test_appending_tiff_writer_writelong(self) -> None:
	data = b"II\x2A\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
	b = BytesIO(data)
	with TiffImagePlugin.AppendingTiffWriter(b) as a:
	a.writeLong(2**32 - 1)
	assert b.getvalue() == data + b"\xff\xff\xff\xff"
	def test_appending_tiff_writer_rewritelastshorttolong(self) -> None:
	data = b"II\x2A\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
	b = BytesIO(data)
	with TiffImagePlugin.AppendingTiffWriter(b) as a:
	a.rewriteLastShortToLong(2**32 - 1)
	assert b.getvalue() == data[:-2] + b"\xff\xff\xff\xff"

Since that is after the assert, the code that is run during close can simply be disabled.

[hugovk]

Please could you add comments to explain why? It won't be obvious in the future without looking up the PR via the blame.

[radarhere]

If that's confusing, then how about a different solution instead - I've updated the code so that it is now seeking backwards through the data first before the write/rewrite operation.