0: Fix Packed unicast size

[jbaumanTT]

Problem description

We're seeing a crash on MeshWorkloadTest.MeshWorkloadOnActiveEth

What's changed

We were always calculating the size with multicast subcommands, which is incorrect. Use the appropriate subcommand type instead.

Checklist

• Post commit CI passes
• Blackhole Post commit (if applicable)
• Model regression CI testing passes (if applicable)
• Device performance regression CI testing passes (if applicable)
• (For models and ops writers) Full new models tests passes
• New/Existing tests provide coverage for changes

[tt-aho]

Do we have unit tests for this case?

[jbaumanTT]

Do we have unit tests for this case?

The MeshDevice tests caught this, but only on T3K, since otherwise there aren't enough active ethernet cores for the mismatch in sizes to appear. In a follow-up I want to move insert_write_packed_payloads into DeviceCommandCalculator so I can unit test it in DeviceCommandTest.