Maximum size of a binary file returned by read() as base64.
Files exceeding this size return a Binary file exceeds maximum preview size
error rather than being base64-encoded in full. Backends overriding read()
should import and reuse this constant to stay in sync with the base
implementation. Kept in lockstep with the MAX_BINARY_BYTES literal in
_READ_COMMAND_TEMPLATE (asserted by test_read_constants_match_template).
