Add fileIds to sendMultimodalMessage

[bsreenivas11]

Summary

• Add fileIds beside the existing fileId on sendMultimodalMessage (plural wins).
• Dual-send file + files on the wire so older backends still receive the first attachment.
• Mark fileId as deprecated. Do not delete it.

Written by Cursor Grok 4.6, using Cursor

Test plan

• Existing multimodal tests updated to expect both file and files
• New plural fileIds test
• CI on this PR
• Confirm an old backend still accepts a payload with both file and files

[cursor]

PR Summary

Low Risk
Backward-compatible extension to a beta conversational AI client API; dual-send reduces breakage for older backends, though wire payload shape changes when attachments are present.

Overview
sendMultimodalMessage now accepts multiple attachments via a new fileIds array, while the singular fileId remains supported but is marked deprecated.

When files are included, the client sends both the legacy file field (first attachment only) and a files array on the multimodal WebSocket event so older orchestrators still see a single attachment while newer backends can consume the full list. fileIds takes precedence when non-empty; otherwise a lone fileId is normalized into a one-element list.

The MultimodalMessageClientToOrchestratorEvent type gains an optional files property, and unit tests cover plural payloads and the dual file/files wire shape.

^{Reviewed by Cursor Bugbot for commit 672315c. Bugbot is set up for automated code reviews on this repo. Configure here.}