textproto: fix boundary prefix collision causing empty part bodies

[mpasquale-3]

Summary

We need the multipart parser to distinguish between closing boundaries (--) and suffixed nested boundaries (-nestedSuffix). Nested multipart boundaries have been seen in real EMLs, and Apple Mail renders the body perfectly.

Claude Summary:
matchAfterPrefix previously returned +1 (boundary match) whenever the character immediately after the boundary prefix was -. The intent was to handle --boundary-- (the closing delimiter), but this also matched --boundary-suffix — a longer, different inner boundary.

In practice: if the outer boundary is S0p-lc86...J and an inner boundary is S0p-lc86...J-5, the outer part body reader encounters --S0p-lc86...J-5\n, matches the outer boundary prefix, sees - as the next character, and returns +1. scanUntilBoundary then reports an empty part body at byte 0, so mdm.Body is nil even though the EML contains HTML content.

Fix

When matchAfterPrefix sees c == '-', peek one more byte to distinguish the two cases:

• Next byte is also - → genuine closing delimiter
• Next byte is anything else → longer boundary, not a match