Build: Declare typing-extensions as a core dependency

[anxkhn]

# Rationale for this change

`typing_extensions` is imported unconditionally at import time, not under a
`TYPE_CHECKING` guard:

- `from typing_extensions import Self` in `pyiceberg/typedef.py`, a module imported
  transitively by nearly the whole package.
- `from typing_extensions import override` in ten catalog and IO modules
  (`pyiceberg/catalog/__init__.py`, `catalog/sql.py`, `catalog/rest/__init__.py`,
  `catalog/glue.py`, `catalog/dynamodb.py`, `catalog/hive.py`, `catalog/noop.py`,
  `catalog/bigquery_metastore.py`, `io/pyarrow.py`, `io/fsspec.py`).

`typing.Self` and `typing.override` are only in the standard library on Python 3.11+
and 3.12+ respectively, while PyIceberg supports Python 3.10 (`requires-python =
">=3.10.0,<4.0.0"`), so the third-party `typing-extensions` back-port is genuinely
required at runtime.

However, `typing-extensions` is declared only in the `dev` dependency group, not in
`[project.dependencies]`, so the distributed package under-declares a required import.
It happens to work today only because `pydantic` (a real runtime dependency) pulls
`typing-extensions` in transitively. If that transitive edge ever changes, a clean
install would fail at `import pyiceberg` with `ModuleNotFoundError: No module named
'typing_extensions'`.

`deptry` (already declared as a dev dependency in `pyproject.toml`) flags exactly this
as `DEP004` ("imported but declared as a dev dependency") at every one of those import
sites.

This moves `typing-extensions` into `[project.dependencies]` with a floor of `4.4.0`,
the release that added `override` (`Self` landed in `4.0.0`, so `4.4.0` is the lowest
version that provides both), and regenerates `uv.lock` accordingly.

# Are these changes tested?

Yes. A regression test `test_typing_extensions_is_a_core_dependency` is added to
`tests/test_version.py`. It reads the installed package metadata via
`importlib.metadata.requires("pyiceberg")` and asserts that `typing-extensions` is
present as a core dependency (a requirement with no environment marker, as opposed to
the marker-gated optional extras). It fails before this change and passes after.

`deptry .` reports zero `typing_extensions` findings after the change (11 before).
`make lint` and `uv lock --check` pass.

# Are there any user-facing changes?

No API or behavior changes. The only user-facing effect is a correctly declared
dependency: installing `pyiceberg` now pulls in `typing-extensions` directly instead
of relying on it arriving transitively through `pydantic`.

---

Notes for the ship step (not part of the PR)

• No tracking issue exists, so no Closes # line. (Historical PR Fix dependency with deptry  #534 proposed the
  same move but was closed by the stale bot for inactivity, not rejected on merit;
  worth mentioning in a PR comment if a maintainer asks, but not required in the body.)
• The changelog label cannot be set by an external contributor (no triage perm);
  this is a build/packaging fix with no user-visible API change, so a changelog entry
  is likely unnecessary. Leave it to maintainer discretion.
• Before opening: rebase patch-15 onto the freshly fetched origin/main (this repo
  moves ~10 commits/1.5 days). If upstream bumped deps, re-run uv lock conflict
  resolution and make lint (uv-lock check) after the rebase.
• Open with: gh pr create -R apache/iceberg-python --base main --head anxkhn:patch-15.