fix: preserve manifest min sequence number of 0

[anxkhn]

Rationale for this change

ManifestWriter.to_manifest_file() computed the manifest's minimum data
sequence number with a truthiness fallback:

min_sequence_number = self._min_sequence_number or UNASSIGNED_SEQ

A data sequence number of 0 is a legitimate minimum for a live file (files
from a v1 table, or the initial commit of a v2 table). Because 0 is falsy,
or collapses it to UNASSIGNED_SEQ (-1), i.e. it treats a real 0 the same
as "unset".

This diverges from the Java reference implementation, which falls back to
UNASSIGNED_SEQ only when the value is actually unset:

// ManifestWriter#toManifestFile (apache/iceberg)
long minSeqNumber = minDataSequenceNumber != null ? minDataSequenceNumber : UNASSIGNED_SEQ;

The -1 is not harmless. When the manifest is produced by a merge/compaction
(so its added_snapshot_id equals the current commit snapshot id),
ManifestListWriter.prepare_manifest() treats min_sequence_number == UNASSIGNED_SEQ as "no file had an assigned sequence number" and overwrites it
with the current, higher commit sequence number
(pyiceberg/manifest.py, the if wrapped_manifest_file.min_sequence_number == UNASSIGNED_SEQ: branch). The manifest's minimum data sequence number is thereby
silently raised, which affects sequence-number-based delete-file application and
scans.

The fix uses an explicit None check instead of a truthiness fallback,
mirroring the Java writer:

min_sequence_number = self._min_sequence_number if self._min_sequence_number is not None else UNASSIGNED_SEQ

Are these changes tested?

Yes. A new parametrized regression test,
tests/utils/test_manifest.py::test_write_manifest_min_sequence_number_zero
(format versions 1 and 2), drives a live EXISTING entry with
sequence_number=0 through ManifestWriter.existing() and asserts that
to_manifest_file().min_sequence_number == 0.

The test fails before the fix (assert -1 == 0) and passes after it. This path
had no prior coverage: no existing test drove a live sequence-0 entry through
to_manifest_file(), which is why the defect was not caught.

Local runs (unit tests):

• pytest tests/utils/test_manifest.py -q -> passes.
• pytest tests/table/test_snapshots.py tests/table/test_manage_snapshots.py -q
  -> passes.
• make lint (ruff, ruff-format, mypy, pydocstyle, codespell) -> clean on the
  changed files.

The merge/compaction integration path exercised by prepare_manifest() is
covered by the integration suite, which requires Docker and Spark; those were
not run locally. The new unit test exercises the same to_manifest_file() write
path that feeds it.

Are there any user-facing changes?

No API changes. It is a correctness fix: manifests written for live files whose
minimum data sequence number is 0 now record 0 instead of -1, so a
subsequent merge/compaction no longer silently raises the manifest's minimum
data sequence number.