fix(offset): correct StartCursor and EndCursor to reference current page items

[josemarluedke]

Summary

Fixes a critical bug in offset-based pagination where StartCursor and EndCursor were pointing to incorrect positions, causing pagination to jump to wrong offsets when navigating between pages.

The Problem

• StartCursor always returned offset 0 (first page of entire dataset)
• EndCursor returned the offset to the last page of the entire dataset instead of the last item on the current page

This caused pagination to jump incorrectly. For example, with 428 total records and page size 10:

• First page would return EndCursor pointing to offset 420 (last page)
• Using that cursor for the second page would jump to offset 420 instead of offset 10
• Result: skipped 41 pages of data

The Fix

Both cursors now correctly reference items on the current page:

• StartCursor: Returns cursor of the first item on current page (currentOffset + 1)
• EndCursor: Returns cursor of the last item on current page, accounting for partial pages at the end
• Both handle empty result sets properly by returning nil

Changes

• Updated buildOffsetPageInfo in offset/paginator.go to calculate cursors based on current page position
• Updated test expectations in offset/paginator_test.go to verify correct cursor behavior
• Improved documentation to clarify cursor semantics

Test Plan

• All 10 offset package tests pass
• All 154 tests pass across entire codebase
• Integration tests with real PostgreSQL pass
• Verified cursor values are correct for first, middle, and last pages
• Verified partial pages (last page with fewer items) return correct EndCursor

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