Provider-response URLs validated; transcription stream envelope added

Patch Changes

• 4be62c1: fix(provider-utils): validate provider-response URLs in getFromApi

  getFromApi now has a validateUrl flag. It is optional so existing callers keep compiling (omitting it behaves like false, i.e. no validation), but all AI SDK provider packages set it explicitly at every call site so each one makes a visible trust decision. When true, the URL is routed through fetchWithValidatedRedirects — the same guard used by downloadBlob — which rejects private/loopback/link-local targets, re-validates every redirect hop, strips proxy/metadata/cookie request headers, and drops all caller headers except the user-agent on cross-origin redirects (custom API-key headers must not follow a redirect off-origin any more than Authorization may); blocked URLs throw DownloadError. It is enabled at the image/video/audio download and polling call sites where the URL comes from a provider response body; URLs built from developer-configured endpoints pass validateUrl: false and are unaffected.

  A new optional credentialedOrigin withholds caller headers unless the URL is same-origin with it, so the API key is not sent to a response-supplied host on a different origin.

  A new optional trustedOrigin exempts URLs (and redirect hops) that are same-origin with the developer-configured provider endpoint from target validation, so self-hosted and localhost deployments whose response URLs point back at the configured host keep working; all other hops are still validated.

  Also closes range gaps in validateDownloadUrl (IPv4 224.0.0.0/4 multicast and the TEST-NET documentation ranges 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24; IPv6 documentation ranges 2001:db8::/32 and 3fff::/20), and follows only the fetch-spec redirect status codes (301/302/303/307/308) — a Location header on any other status is not followed. This guard performs string/literal checks only and does not resolve DNS; hostnames that resolve to private addresses and DNS rebinding remain out of scope and must be constrained at the network layer (or by injecting a Node fetch that pins the resolved IP at connect time) for server deployments handling untrusted URLs. See contributing/secure-url-handling.md.

• 7805e4a: Add experimental transcription-stream WebSocket envelope (standard doStream-over-WebSocket serialization): frame type constants, experimental_parseTranscriptionStreamClientFrame, experimental_serializeTranscriptionStreamPart, and experimental_parseTranscriptionStreamPart (all APIs are exported with experimental prefixes). serializeTranscriptionStreamPart returns undefined for payloads that are not JSON-serializable (callers drop the frame) and serializes cross-realm Error payloads by brand check.

• cd12954: Reject empty OpenAI, Anthropic, and Replicate base URLs with a helpful AI SDK invalid argument error.