test: improve processing flags mutation coverage

[Pissaro007]

Summary

This PR improves mutation coverage for processing_flags.cpp by strengthening the existing GuidanceTest.GuidanceMessages test.

Changes

• Added coverage for flags spanning multiple bytes.
• Added coverage for the maximum allowed flag.
• No new test was created.
• No production code was modified.
• No test was excluded.
• Mull execution and test-identification workflows are intentionally excluded from this PR.

Mutation testing results

• Generated mutants: 16
• Initially surviving mutants: 6
• Mutants killed after test modification: 4
• Remaining mutants: 2
• Final raw mutation score: 87% (14/16)
• Adjusted mutation score excluding equivalent mutants: 100% (14/14)

Remaining equivalent mutants

1. processing_flags.cpp:22:52
   Replaced / with * (cxx_div_to_mul)

   This mutation increases the allocated buffer size without changing observable behavior.

2. processing_flags.cpp:49:32
   Replaced < with <= (cxx_lt_to_le)

   The additional iteration checks bit 8 of an 8-bit value and cannot trigger an additional callback.

Validation

The existing test suite and Mull mutation campaign pass with four additional mutants killed.

Target branch

quality/mull-mutation-testing-poc