[NAV#707] Fix SC unit tests to work with large SC_PACKET_MAX_SIZE

[thesamprice]

• SC_LoadAts_Test_AtsEntryOverflow: Use fixed 256-byte message size to prevent stack buffer overflow
• SC_UpdateAppend_Test_CmdDoesNotFitBuffer: Use dynamic values based on MaxBufEntries
• SC_UpdateAppend_Test_EndOfBuffer: Fix word count calculation for variable-sized entries

These fixes make the tests robust against SC_PACKET_MAX_SIZE values up to 4096 bytes. All 150 unit tests now pass.

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