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fix: match real engineering-tool SZL read and continuation request layouts - #942
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Fixes #937.
Replaying captures of real S7-300 controllers showed two byte-level differences from what an engineering tool sends:
build_read_szl_requestused the data header0a 00(a request without data) in front of the SZL id and index. Real tools, and native Snap7 for requests that carry data, send return code0xFFwith octet-string transport size0x09.build_userdata_followup_requestused an 8-byte parameter block with method0x11. Real continuation requests use 12 bytes with method0x12(00 01 12 08 12 <type|group> <subfn> <seq> 00 00 00 00).Both are changed; the follow-up builder is shared, so block-info and list-blocks continuations get the same form. The bundled server did not depend on the old bytes. Tests pin the real SZL read request for SZL 0x0132, index 4 (ignoring the PDU reference) and the new follow-up layout.
I could not confirm whether a PLC rejects the old requests; this aligns the packets with what the captures show a real tool sending.