fix: check CFE_SB_Subscribe return values in TO_LAB_init

[stark256-spec]

Summary

Both CFE_SB_Subscribe() calls in TO_LAB_init() silently discard their return status. A failure means the application will not receive commands (TO_LAB_CMD_MID) or housekeeping requests (TO_LAB_SEND_HK_MID), yet it continues initialising and reports success to CFE_ES — making the fault invisible at startup.

Fixes #229.

Changes

• Capture the return value of each CFE_SB_Subscribe() call and assign it to status
• Emit TO_LAB_SUBSCRIBE_ERR_EID on failure, matching the error-handling style used throughout the rest of TO_LAB_init()
• Split into two separate if (status == CFE_SUCCESS) blocks so a failure in the first prevents the second from masking it

Before / After

// BEFORE — return values silently ignored
CFE_SB_Subscribe(CFE_SB_ValueToMsgId(TO_LAB_CMD_MID), TO_LAB_Global.Cmd_pipe);
CFE_SB_Subscribe(CFE_SB_ValueToMsgId(TO_LAB_SEND_HK_MID), TO_LAB_Global.Cmd_pipe);

// AFTER — captured and reported
status = CFE_SB_Subscribe(CFE_SB_ValueToMsgId(TO_LAB_CMD_MID), TO_LAB_Global.Cmd_pipe);
if (status != CFE_SUCCESS)
    CFE_EVS_SendEvent(TO_LAB_SUBSCRIBE_ERR_EID, CFE_EVS_EventType_ERROR,
                      "L%d TO Can't subscribe to Cmd MID, status %i", __LINE__, (int)status);