fix: add validation for nested_format in Querystring

[omkute10]

Description

This PR enhances the Querystring class by adding proper validation for the nested_format parameter in _stringify_item(). The changes ensure that only valid nested format values ("dots" or "brackets") are accepted, improving code robustness and developer experience.

Changes

• Added input validation for nested_format parameter at the start of _stringify_item()
• Improved error messages to clearly indicate valid format options
• Restructured key formatting logic for better readability and maintainability
• Added type safety with get_args(NestedFormat) for future maintainability
• Included defensive programming with an unreachable code path check

Testing

The following test cases should be verified:

• Using nested_format="dots" works as expected
• Using nested_format="brackets" works as expected
• Invalid nested_format values raise a clear ValueError
• Existing functionality for other parameters remains unchanged

Impact

• Security: Prevents potential issues from invalid format values
• Developer Experience: Clear error messages help identify issues faster
• Maintainability: Better structured code with improved type safety
• Future-Proof: Easier to add new nested formats if needed

Related Issues

• Resolves TODO comment about handling unknown formats
• Follows up on type safety improvements from previous PRs

Additional Notes

• The changes are backward compatible
• No breaking changes introduced
• Documentation updates not required as this is an internal method