fix(chat): fail closed when attachment MIME sniff sees invalid UTF-8 binary

Post-merge forensic from PR #961 exact head 1115421a85e19420a1fdc6a03c3fc6797dd994de. attachment-mime-sniff.ts currently treats arbitrary no-NUL bytes as text. Counterexample bytes 255,254,253,252,251,250 are classified as family=text and acceptedAsText=true. Required fix: validate UTF-8/text semantics and fail closed for binary payloads, with mutation/negative tests. One issue, one PR. Related governance incident: #956. Coordination state: recorded but not dispatched because it is not in the current pnpm harness readiness queue.

Start in attachment-mime-sniff.ts and reproduce the counterexample with bytes 255, 254, 253, 252, 251, and 250. Validate UTF-8/text semantics so invalid binary payloads fail closed, then add mutation and negative tests showing they are not accepted as text.

Written by the indexing model from the issue text.