Support findMany offset via cursor pagination

[MauruschatM]

Motivation

findMany throws offset not supported, but some Better Auth surfaces pass offset — e.g. the dashboard's table paging. This PR supports it and re-enables the five upstream offset tests in @better-auth/test-utils.

Changes

The component paginates by cursor and stays cursor-only. The client fetches offset + limit rows through the existing cursor pagination and drops the first offset rows; offset is stripped from the component query args.

Cost

• offset + limit reads exactly offset + limit rows — deep offsets cost proportionally, the same as any cursor-based store emulating offset. This is aimed at admin-style paging.
• offset without limit can't be rejected (three of the four upstream offset tests pass offset with no limit), but it's cost-equivalent to the already-supported no-limit findMany, which pages 200 rows at a time to the end — no new unboundedness is introduced.

Tests

• Re-enabled the upstream offset tests. The three sortBy and offset variants mutate runtime schema (numericField additional field), so they follow the existing pattern: added to ADDITIONAL_FIELDS_NORMAL_TESTS and run in the additional-fields profile, which has the static field + index.
• New convex-custom test: offset applied to OR unions.

npm run build, npm run typecheck (src), and the full vitest suite pass: 10 files, 188 passed | 26 skipped (main baseline: 182 passed | 31 skipped).

Note: this touches the same findMany region as #404; whichever merges second will be rebased promptly.

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