Reject ISO 8601 durations with missing components

[shkyyy18]

Summary

Env.timedelta(..., format="iso8601") currently accepts duration strings without any components (P, PT, signed/whitespace variants) as zero. It also accepts an empty time section after a day component (P1DT becomes one day). This silently accepts malformed configuration instead of raising EnvValidationError.

Require a component after P, and a time component whenever T is present. Valid zero durations such as P0W, P0D, and PT0S remain supported. The change is limited to two regex lookaheads; invalid strings fall through to the existing validation path.

Regression coverage

• Nine malformed-duration cases: all fail on the unchanged upstream implementation and pass with the fix.
• Six explicit-zero controls: pass both before and after.
• No new dependency or API change.

Validation

Python 3.12.10, marshmallow 4.3.1, dependencies from the existing frozen lockfile:

• New ISO 8601 regression cases: 15 passed.
• Complete suite: 149 passed, 1 failed. The failure is the pre-existing Windows-only behavior of test_read_from_file_path_is_unreadable: chmod(0o000) does not make the file unreadable here. On the unchanged upstream checkout, the same test fails (134 passed, 1 failed). No unrelated permission test was modified.
• Excluding only that baseline permission test: 149 passed, 1 deselected.
• mypy --warn-unused-ignores: passed.
• Configured pre-commit hooks on both changed files: Ruff check/format and blacken-docs passed; workflow, lockfile, and README-only hooks had no applicable files.
• git diff --check: passed.

I have not run the full upstream multi-version/Linux CI matrix locally.

AI assistance was used to investigate, implement, and test this change.