Fix validation of FC22 echoed confirmation fields

[Daniiiil1]

What changed

• add a dedicated FC22 confirmation case in check_confirmation()
• require the echoed reference address, AND mask, and OR mask to match the request
• add independent integration regressions for a mismatch in each echoed field

Why

The Modbus specification defines the normal Mask Write Register response as an exact echo of the request fields. FC22 previously fell through to the generic one-value case, so a response with the correct function and length could be accepted even when the echoed address or masks differed.

Fixes #868.

Validation

• git diff --check
• ./autogen.sh
• ./configure --disable-shared
• make -j4
• tests/unit-tests.sh — ALL TESTS PASS WITH SUCCESS

One earlier full-suite run hit the pre-existing timing-sensitive 7 ms byte-timeout test on macOS; the FC22 regressions passed in that run, and a clean rerun of the complete integration script passed.

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