Document collectibles spam filtering flags

[shaneauerbach]

Resolves API-3327

Summary

• clarify default behavior for filter_spam and show_spam_scores
• document that is_spam is always present while scores/explanations are gated
• align spam feature names and value types with the API response

Test plan

• Not run (docs-only changes)

[cursor]

PR Summary

Collectibles spam filtering and scoring docs/schema alignment

• Clarifies default behavior of filter_spam and show_spam_scores; is_spam always present; enabling scores does not disable filtering
• Updates spam scoring signals: rename ipfs_uri_present→uri_non_ipfs, url_shortener_in_uri→url_shortener, add manual_classification; refine descriptions
• Expands explanations.value to allow string in OpenAPI and adjusts descriptions to list returned fields (spam_score, explanations)
• Fixes docs anchor in filter_spam description link (#spam-filtering)

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