fix(server): application/octet-stream file attachments crash the session #35697

Bug description

When a client sends a file attachment with mime: "application/octet-stream" (unrecognized MIME type), the server pushes it as a file part that reaches provider converters. The converters only handle image/* (and application/pdf in responses), so they throw UnsupportedFunctionalityError, crashing the session.

Reproduction

1. Send a file with unknown/unspecified MIME type via any client (e.g. drag a binary file without recognized extension)
2. The server gate at message-v2.ts:212 converts it to a file part
3. The copilot converter receives mediaType: "application/octet-stream" and throws

Root cause

PR #23501 (custom provider image support) set image: true capability, which let non-image file parts flow through the AI SDK to converters that can only handle images.

Expected behavior

Non-media file parts should be converted to text placeholders at the gate level, and converters should silently skip unrecognized file types instead of crashing.