bug: replace `strptime()` with ISO parsers for date/time parsing

[nos1dot618]

The _parse_date, _parse_datetime, and _parse_time helpers use datetime.strptime(), triggering Ruff DTZ007 due to naive datetime construction.

Replace the manual format loops with the appropriate fromisoformat() APIs to:

• Remove the DTZ007 warnings.
• Simplify the parsing logic.
• Preserve the existing supported date, datetime, and time formats.

Add/update regression tests to verify the existing parsing behavior remains unchanged.

[github-actions]

Hi Lakshay Chauhan (@nos1dot618), thank you for opening this issue!

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[nos1dot618]

After some digging, I found that date.fromisoformat() is stricter than the previous strptime(..., "%Y-%m-%d") behavior. The previous implementation accepted dates with single-digit month/day components, such as 2024-5-2, whereas fromisoformat() rejects them and returns None.

Since this changes previously supported behavior and the goal is to maintain compatibility, I think it would be better to retain the existing functionality by normalizing the components before calling fromisoformat() (or retaining a compatibility fallback), along with a regression test for this case.