Reject datetime strings in the date factory

[jadenfix]

Summary

tomlkit.date() currently accepts a local or offset timestamp and returns a DateTime, because Python's datetime subclasses date. Reject datetime values with the existing date-only ValueError, preserving valid dates and the separate datetime() factory.

Adds regressions for local, UTC, and non-UTC offset timestamps. All three fail before the fix. The complete suite passes: 1,206 tests on Python 3.12.13, including the pinned toml-test submodule. All repository pre-commit hooks pass; the local pre-commit runner was upgraded because the declared 2.20 version cannot parse the current hook manifest's stage names. An installed-wheel consumer round-trips a real date/datetime TOML document through stdlib tomllib and checks timestamp rejection. Other interpreter versions and upstream CI are not yet verified.

The earlier #645 addressing this behavior was self-closed unmerged; this change was developed from the current factory and its date-only contract.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-6-based Codex; the exact deployment model identifier is not exposed in this session
• Notes: Codex prepared the implementation, regression tests, and this description. A separate Codex agent independently reviewed exact commit 5ef0158 and ran the seven date-related API tests, with no unresolved findings. This is automated review, not human maintainer review.