Return deferred search content in Go SDK

[chruffins]

Summary

• return the generated search contents response from Search.Contents.Fetch
• add generated response/result models and refresh search request docs

Testing

• go test . -run TestSearchContent
• go vet ./...

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Note

Medium Risk
Breaking change to Fetch's signature and behavior (typed response vs error-only), plus updated semantics for browser-backed retrieval and billing that integrators must follow.

Overview
Enables deferred search result content retrieval in the Go SDK: Search.Contents.Fetch now unmarshals and returns *Response (contents, search_id, usage, warnings) instead of only signaling success with error. New generated types include Response, ResponseContent, and ResponseContentError.

This replaces the previous stub that documented a 404-only placeholder. Callers must update to handle the new return value (res, err := ...). api.md documents the Response type and updated method signature; the integration test discards the response with _.

Search content option and result field docs are refreshed across search.go and searchcontent.go to match the API: retained provider content vs browser fetch for source (auto / provider / browser), max_age_hours semantics, temporary browser provisioning and 429 partial behavior for source=auto, and clarified cache_status / fetched_at / usage content_fetches wording.

^{Reviewed by Cursor Bugbot for commit 3b32fce. Configure here.}

[chruffins]

Superseded by the Stainless-generated release PR #193, which includes this SDK update.