Add MIME hints to provider-file attachments (FIG-616 item 2)

[SamGalanakis]

AttachmentSource::ProviderFile was the only variant carrying no MIME (Inline, ExternalUrl and Stored's AttachmentRef all have one). Because source.media_type() returned None for it, the Anthropic adapter had an early return hardcoding a document block — so an image file id produced a wire error or the wrong modality, since Anthropic requires image file ids in an image block.

Design (grilled and ratified)

• media_type: Option<MediaType> — optional, not required. Verified that only Anthropic needs it: OpenAI validates the provider scope and emits by file id, Google emits {"fileData":{"fileUri":id}}; both resolve the type server-side. Requiring it would tax the two providers that never consume it and invite invented values.
• Open vocabulary preserved. No closed MIME enum — MediaType is deliberately a validated newtype over String ("Lash deliberately does not maintain a closed media catalog", FIG-564), and a closed catalog would contradict that and need a lash release per new provider MIME.
• Anthropic: the fix is a deletion. The hardcoded-document early return is gone; provider file ids now flow through the normal is_image() dispatch. When the hint is absent it fails before the network (provider_file_media_type_required, Validation kind) rather than silently guessing document — unknown stays representable for the providers that don't care, but can never produce the wrong modality.

Verification

Type size 104 B, under the existing size_of <= 128 compile-time ceiling (unchanged). Serde omits the field when None, so existing payloads deserialize; core↔remote round-trip preserves both Some and None. Tests cover image/document block selection, the absent-hint refusal, and serde compatibility — independently re-run, 12/12 green. OpenAI and Google behaviour unchanged.

Breaking at source level only: the provider_file helper gains a third argument and direct enum construction gains a field; serialized payloads are unaffected.

Closes FIG-616 item 2. Item 3 was separately ratified as keep all 12 OpenAI MIMEs — OpenAI's live docs disprove the PDF-centric premise, and our list is in fact a subset of what they accept.