refactor: Code simplification and helper extraction

[josemarluedke]

Summary

This PR simplifies the codebase by extracting helpers, eliminating duplication, and improving readability. All changes are non-breaking and preserve existing functionality.

Changes

1. File Organization

• Split models.go into page_args.go and page_info.go for better organization
• Added NewEmptyPageInfo() constructor for convenient empty PageInfo creation

2. Helper Extraction

cursor/paginator.go

• Extracted parsePageArgs() helper to consolidate duplicate validation logic
• Both New() and BuildFetchParams() now share this code
• Removed dead code: unused defaultOrderBy and buildOrderBy() function
• Saved: ~50 lines

cursor/schema.go

• Extracted fixedFieldsToSorts() helper method
• Eliminated repeated fieldSpec-to-Sort conversion code
• Saved: ~25 lines

quotafill/quotafill.go

• Extracted argsForEncoder() and getSortBy() helpers
• Eliminated 4 instances of duplicated cursor args building
• Saved: ~30 lines

3. Code Simplification

offset/cursor.go

• Simplified DecodeCursor() with early return pattern
• Clearer control flow, removed unnecessary variable pre-declarations

sqlboiler/cursor.go

• Consolidated pass-through switch cases
• Reduced: 15 lines → 3 lines

4. Test Coverage

• Added comprehensive test for NewEmptyPageInfo()
• Verifies all fields return correct nil/false values

Impact

• ✅ Net reduction: ~50 lines of code (108 insertions, 186 deletions)
• ✅ All tests passing: 175/175 specs (added 1 new test)
• ✅ No breaking changes: All public APIs preserved
• ✅ No behavior changes: Functionality remains identical
• ✅ Improved maintainability: Less duplication, clearer code organization

Commits

1. feat(core): add NewEmptyPageInfo and refactor file organization
2. refactor: extract helpers and simplify code across packages
3. test(core): add test coverage for NewEmptyPageInfo

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