test(isb): improve shortcut button mutation coverage

[Pissaro007]

Summary

This pull request enriches the existing ISB shortcut button tests to improve mutation coverage for isobus_shortcut_button_interface.cpp.

Changes

• Enriched IsobusShortcutButtonTest.ShortcutButtonTxTests.
• Killed the line 67 cxx_eq_to_ne mutant.
• Killed the line 140 cxx_post_inc_to_post_dec mutant.
• Enriched IsobusShortcutButtonTest.ShortcutButtonRxTests.
• Killed the line 197 cxx_eq_to_ne mutant.
• Preserved the existing production code and GoogleTest declarations.
• Excluded the experimental mutation-testing workflows from this PR.

Results

• Initial mutation score: 77%
• Final raw mutation score: 94%
• Killed mutants: 17/18
• Remaining mutant: line 179 cxx_lt_to_le
• The remaining mutant is considered likely equivalent.
• Adjusted mutation score: 100%

Validation

• Targeted ISB Rx and Tx tests pass.
• The complete unit test suite passes.
• Mull confirms that the mutants on lines 67, 140 and 197 are killed.
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