fix(messaging): pass through PDF attachments

[huntharo]

Summary

• I changed messaging PDF ingestion to pass PDFs through as native file input instead of extracting PDF text.
• I added shared file input/message-part contracts and wired Codex, Grok, replay persistence, and transcript rendering around file metadata.
• I updated the messaging docs and added focused coverage for PDF pass-through, AI SDK file parts, controller routing, and replay sanitization.

Verification

• I ran pnpm test apps/desktop/src/main/__tests__/messaging-attachment-processor.test.ts apps/desktop/src/main/__tests__/messaging-controller.test.ts packages/agent-core/src/__tests__/ai-sdk-message-builder.test.ts packages/agent-core/src/__tests__/session-state.test.ts.
• I ran pnpm typecheck.
• I ran pnpm lint:boundaries.

Notes

• This keeps PDF bytes in turn input only; replay/transcript persistence stores file name, MIME type, and size metadata, not the base64 PDF payload.
• Follow-up issue for optional PDF-to-image preprocessing: feat(messaging): render inbound PDFs to images before model upload #385.

[huntharo]

Addressed the Codex protocol concern from review. I checked the installed Codex Desktop bundle: non-image files are treated as local file references, with prompt context listing the saved paths, rather than raw file UserInput variants. This branch now mirrors that at the Codex adapter boundary: file inputs are written to a temp path and sent as text context + a mention, so turn/start never includes an unsupported type: file item. Grok still receives native file inputs.