feat(ab-testing): Add pagination to getAllExperiments() endpoint (#1390)

[chrissarah054-dotcom]

closes #1390
BREAKING CHANGE: Response format changed from array to paginated object

• Replace unbounded find() with paginated createQueryBuilder() + getManyAndCount()
• Remove eager loading of variants/metrics from list view (performance optimization)
• Add PaginationQueryDto support for page, limit, sortBy, order parameters
• Implement SQL injection prevention via sortBy field whitelist validation
• Return OffsetPaginatedResponse with metadata (total, page, hasNextPage, etc.)
• Configure default page size: 10 items, max: 100 items per PAGE_SIZE constants

Performance improvements:

• Query time: O(n) -> O(1) constant regardless of total experiments
• Memory: O(n×m) unbounded -> O(limit) bounded by page size
• Response size: Unbounded -> Capped at 100 items maximum
• Database: Eliminated N+1 query problem via single getManyAndCount()

Database optimization:

• List view (getAllExperiments): No relation eager loading for bounded queries
• Detail view (getExperimentById): Maintains full relations (variants, metrics)
• Indexed createdAt field provides fast sorting and offset pagination

API Changes:
Before: GET /ab-testing/experiments -> Experiment[]
After: GET /ab-testing/experiments -> OffsetPaginatedResponse

Supported query parameters:

• page (1-based, default: 1)
• limit (1-100, default: 10)
• sortBy (createdAt, updatedAt, name, status, startDate; default: createdAt)
• order (ASC, DESC; default: DESC)

Tests:

• Added 8 comprehensive pagination test cases
• Coverage: default pagination, skip/take math, sorting, SQL injection prevention, metadata calculation, no eager loading verification, page size clamping, edge cases

Docs:

• Full Swagger/OpenAPI documentation with examples
• Implementation summary with migration guide
• Performance comparison (before/after metrics)

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[RUKAYAT-CODER]

Well done on the job done so far!
Kindly fix workflow to pass