feat(ebay): expose pagination.has_more; is_sponsored is now nullable

[0xGhostCasper]

Tracks scrape-badger/scrapebadger#609.

Pagination.has_more — the stop signal for bulk extraction of completed/sold listings. The model uses extra="ignore", so without this field the SDK silently dropped it and users could not page to the end: total_pages/total_results are null on the sold grid, and past the last page eBay re-serves that page instead of returning empty.

page, sold = 1, []
while True:
    r = client.ebay.completed(query="iphone 13", per_page=240, page=page)
    sold += r.results
    if not r.results or not r.pagination.has_more:
        break
    page += 1

SearchResult.is_sponsored — now bool | None, defaulting to None. eBay renders its "Sponsored" badge into every card as anti-scraping bait, so the API reports null rather than true for 100% of results.

Both are additive/widening: existing code reading is_sponsored still type-checks against bool | None consumers, and no field was removed.

[codecov]

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