Fix adding/updating todo items with due date in CalDAV integration

[DerFlob]

Proposed change

Fixes #105246.

Before the due_datetime was manually converted to a caldav-compatible date string, when adding or updating a todo. However the method to save a new todo expects a python-native date/datetime.

Additionally, when updating an existing todo, there is a helper method to convert the date/datetime to the appropriate date string.

Tested adding and updating against Nextcloud/SabreDAV.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes todo.add_item service fails on CalDAV integration with due_datetime set #105246

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
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• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.

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• Documentation added/updated for www.home-assistant.io

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[allenporter]

Thank you for the fix, appreciate it.

[allenporter]

The changes look good, but there are some mypy/ruff errors when running on CI. Can you take a look?

[DerFlob]

The changes look good, but there are some mypy/ruff errors when running on CI. Can you take a look?

For some reason the mypy pre-commit hook on my side complains, when the # type: ignore comment is there. Which is why I had them deactivated. 😅

[allenporter]

The changes look good, but there are some mypy/ruff errors when running on CI. Can you take a look?

For some reason the mypy pre-commit hook on my side complains, when the # type: ignore comment is there. Which is why I had them deactivated. 😅

Perhaps its a different in local mypy version compared to the one running in CI? Anyway, thanks.

[DerFlob]

The changes look good, but there are some mypy/ruff errors when running on CI. Can you take a look?

For some reason the mypy pre-commit hook on my side complains, when the # type: ignore comment is there. Which is why I had them deactivated. 😅

Perhaps its a different in local mypy version compared to the one running in CI? Anyway, thanks.

Probably something like that. I only set up the dev env quickly yesterday.
Thank you for merging! :)