Preserve four-digit years when parsing day-of-year formats

[shguild]

from_format() cannot round-trip day-of-year formats for years before 1000. For example, pendulum.from_format("0001-031", "YYYY-DDDD") raises ParserError: Unable to parse string [1-031], although the original string is valid. The formatter converts the year to an integer and then reconstructs the ISO ordinal date without padding it back to four digits.

Keep the year zero-padded when constructing that intermediate date. Add coverage for DDD and DDDD, the minimum and maximum years, a leap day, the year 1000 boundary, time/offset preservation, and invalid dates. The new 16 cases produce 9 failures and 7 passes on the original code.

Validation on Windows / Python 3.12.14:

• Pure Python full suite: 1884 passed, 5 skipped.
• Current Python sources with the official 3.2.0 Rust extension: 1884 passed, 5 skipped. The Rust sources were not rebuilt locally.
• mypy: success, 228 source files.
• Ruff 0.14.11 lint and format checks pass for both changed Python files.
• git diff --check passes.

Pull Request Check List

• Added tests for changed code.
• Updated the changelog for the bug fix.

Prepared with Codex (GPT-6) assistance.