A safetensors length prefix such as 640 encodes as 80 02 00 ..., which the
provider read as a pickle PROTO marker and refused before the safetensors
parser ran. The parser that owns the length prefix now makes the call: a
pickle-like prefix is safetensors only when byte 8 is '{' and the declared
length is within the safetensors header ceiling. Real pickles stay refused
and are never deserialized.
Fixes #5895