Fix exception function code wraparound when function >= 0x80

[afonsojanu]

response_exception() builds the exception function byte with plain integer addition:

sft->function = sft->function + 0x80;

modbus_reply() routes any function code its switch doesn't recognize through the default case into this function, and that includes raw bytes from 0x80 to 0xFF (a client can send those directly with modbus_send_raw_request, or they show up as garbage/unsupported function codes on the wire). When sft->function is already >= 0x80, the addition carries past 0xFF. sft->function is an int, so the overflow itself is harmless, but the value then gets written into the response buffer as a single byte in build_response_basis (rsp[7] = sft->function; for TCP, similarly for RTU), which truncates it back into range. For a function byte like 0xFF, 0xFF + 0x80 = 0x17F, which truncates to 0x7F, a value with the top bit clear. The client then sees what looks like an ordinary function code instead of an exception.

This is the same class of bug as #845, which covers modbus_reply_exception() (already has an open PR, #869). This report is about the separate code path inside modbus_reply() itself, used both for the "unrecognized function code" case and the "truncated request" case, which #869 doesn't touch.

Fix: use bitwise OR instead of addition, matching the actual Modbus rule (set the top bit of the function code). For any function code below 0x80 this produces exactly the same byte as before; for one at or above 0x80 it now stays correct instead of wrapping.

Verified with a small standalone repro that opens a modbus_reply() call through a socketpair and inspects the raw response bytes: with a request function byte of 0xFF, the reply's function byte came back as 0x7F before this change and 0xFF after.

Also adds a client-side test next to the existing "invalid function code" test in unit-test-client.c, using a function byte that already has its top bit set (0xFF), which fails before the fix and passes after.

[cla-bot]

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