MAINT consolidate remote datetime parsing

[Roman Lutz (romanlutz)]

Description

Remote dataset loaders for 0DIN and PromptIntel duplicated the same ISO 8601 datetime parsing logic. This moves that behavior into _RemoteDatasetLoader so remote loaders share one typed implementation while preserving the existing handling of UTC Z suffixes, explicit offsets, empty values, and invalid input.

Tests and Documentation

• Added deterministic unit coverage for the timestamp formats returned by both APIs and for empty or invalid input.
• Ran the focused remote-loader, 0DIN, and PromptIntel unit tests: 92 passed.
• Ran focused Ruff check and format validation.
• Documentation was not changed because this internal consolidation does not affect the public API or usage.