fix: accept exhausted WHOOP collection pages

[Aarekaz]

Root cause

WHOOP's live v2 collection API returns a null next-token value when a page is exhausted, while its published OpenAPI describes the field as absent. The strict parser rejected all cycle, recovery, sleep, and workout pages before persistence.

Fix

• accept null or absent provider next tokens
• normalize both to internal undefined
• retain opaque string tokens unchanged for real continuation pages

Verification

• production-safe diagnostics reproduced next-token invalid-type errors for all four collections
• RED regression rejected a null next token
• GREEN focused 67/67, full API 286/286, typecheck, diff check