Fix undefined object p in metamodel parameter validation

[jurjoroa]

Goal

metamodel()'s invalid-parameter branch computes setdiff(p, pnames), but p is never defined in that scope — it only exists as a loop variable in the oneway branch further down. Passing a parameter name absent from the PSA therefore raises object 'p' not found instead of the intended informative message, and the same path is reachable through calc_evppi() and calc_evsi().

Scope

• Change setdiff(p, pnames) to setdiff(params, pnames) at R/metamodel.R:59
• Add a regression test asserting the intended message when params names a parameter absent from the PSA
• Confirm the corrected message also surfaces when the path is reached via calc_evppi() and calc_evsi()

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Version: #172