Fix CMD_SEND_CHANNEL_DATA payload validation

[VoxMesh]

Fixes #3345

Validate CMD_SEND_CHANNEL_DATA payloads against MAX_GROUP_DATA_LENGTH instead of
MAX_CHANNEL_DATA_LENGTH.

This rejects 166-167 byte payloads with ERR_CODE_ILLEGAL_ARG before they reach
sendGroupData(), while leaving the receive-side MAX_CHANNEL_DATA_LENGTH check unchanged.

Verified:

• 164-165 byte payloads pass the validation check
• 166-168 byte payloads are rejected by the validation check
• Heltec_v3_companion_radio_ble builds successfully