fix: build StaticTable version-hint metadata locations with forward slashes

[anxkhn]

Rationale for this change

StaticTable._metadata_location_from_version_hint builds the version-hint.text
location and the resolved metadata-file location with os.path.join:

version_hint_location = os.path.join(metadata_location, "metadata", "version-hint.text")
...
return os.path.join(metadata_location, "metadata", content)

Iceberg locations are URIs (s3://, gs://, abfss://, file://, ...) and are
always forward-slash separated. os.path.join uses the host OS separator, so on
Windows it produces backslash-separated keys such as
s3://warehouse/wh/nyc.db/taxis\metadata\version-hint.text. That is an invalid
object-store key, so FileIO.new_input(...) cannot find the file and the lookup
fails.

This breaks the documented "pass the table root path" usage of
StaticTable.from_metadata on Windows. The API docs advertise exactly this path:

If your table metadata directory contains a version-hint.text file, you can
just specify the table root path, and PyIceberg will read the version hint file
and load the latest metadata file.
StaticTable.from_metadata("s3://warehouse/wh/nyc.db/taxis")

This is the only place in the codebase that builds a table/metadata location
with os.path.join; every other location builder already uses explicit forward
slashes (for example pyiceberg/table/locations.py, and the metadata-path helpers
in pyiceberg/catalog/__init__.py). This change makes
_metadata_location_from_version_hint consistent with them:

metadata_dir = f"{metadata_location.rstrip('/')}/metadata"
version_hint_location = f"{metadata_dir}/version-hint.text"

Behavior is unchanged on POSIX. The now-unused os import (this was its only use
in the module) is removed to keep the linter happy.

Are these changes tested?

Yes. tests/table/test_init.py gains
test_static_table_version_hint_location_uses_forward_slashes, a regression test
that is deliberately OS-independent: it swaps os.path.join for ntpath.join
(the Windows join) via monkeypatch, stubs load_file_io with a small fake
FileIO, and asserts the resolved metadata location stays a valid forward-slash
URI (.../metadata/version-hint.text and .../metadata/<file>.metadata.json,
with no backslash). It is parametrized over the three version-hint content forms
the method handles (full *.metadata.json filename, a numeric version, and a
non-numeric value). It fails on the old os.path.join code (backslash separators)
and passes after this change, on any platform.

The pre-existing test_static_table_version_hint_same_as_table only exercises a
local temp path, where the separator is already / on Linux CI, so it never
caught this. Windows is not yet in CI (tracked separately in #2477), which is why
the OS-independent approach is used here.

• pytest tests/table/test_init.py -k version_hint -> passes (existing + 3 new cases).
• pytest tests/table/test_init.py -> passes.
• pytest tests/test_serializers.py -> passes (exercises the real
  from_metadata -> _metadata_location_from_version_hint path over local files).
• make lint -> clean.

The integration suite (Docker + Spark) was not run locally; this change is covered
by the unit tests above and does not alter POSIX behavior.

Are there any user-facing changes?

No API or signature changes. This is a bug fix: StaticTable.from_metadata(<table root>) and StaticTable._metadata_location_from_version_hint now build correct
forward-slash metadata URIs on Windows. POSIX behavior is unchanged.

Related: #2477 (infra: run tests for windows).