Add default error branch to getParamCCP() for unknown dataset keys

[Copilot]

getParamCCP() had no fallback for unrecognized data values, leaving object and column variables undefined and producing confusing downstream errors.

Changes

• tests/testthat/helper-Compare_Cell_Populations.R: Added an else branch that stop()s immediately with a clear message enumerating the supported dataset keys:

} else {
  stop(paste0(
    "Unknown dataset '", data, "'. ",
    "Supported datasets are: TEC, Chariou, PBMC, NSCLC, BRCA."
  ))
}

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