fix(bridle): tell the model a binary attachment exists (CLEAN-57)

[maksymhryzodub-prog]

Summary

• Follow-up to feat(bridle): accept Office documents as chat attachments (CLEAN-57) #56, which was merged before this commit landed on its branch — re-targeted onto fresh main.
• The runtime drops file parts before the model call, so a binary attachment (PDF, Office) was invisible to the agent: it denied seeing the file the person just sent, and an attachment-only message reached the model as an empty turn ("What can I help you with?").
• expand() now inlines a short notice — file name, MIME type, size, and that the contents are not readable — using the same mechanism as text-file inlining. The agent answers honestly: "I see haha.xlsx but can't read its contents."
• Also applies to text-typed files downgraded to binary (undecodable bytes): previously silent, now acknowledged.

Jira: CLEAN-57

Test plan

• api jest: 255 pass (binary expansion inlines the notice, attachment-only message gets a text body, downgraded files acknowledged, contents never inlined)
• After deploy: send an .xlsx with "что в файле?" to the prod Rancher — expect the agent to name the file and say it can't read it

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