[Prem-gosavi45]

Closes #2014

Make IntervalDtype generic over its closed parameter to preserve literal types when accessing the closed property.

• Add a type variable for the closed side, with a default that preserves existing typing behaviour.
• Update the constructor and closed property annotations.
• Add type assertions covering all four closed values: "both", "left", "right", and "neither".
• Preserve the default constructor typing behaviour.

Validation:

• poetry run poe test — passed
• 3510 passed, 5 skipped
• 62 architecture tests passed
• All type checkers and pre-commit hooks passed.