Add tests for invalid bounds in density plotting functions

[Copilot]

Adds test coverage for error handling in validate_density_bounds across density plotting functions, addressing review feedback from #405.

Changes

• test-ppc-distributions.R: Added "density PPC/PPD plots reject invalid bounds" test block
• test-mcmc-distributions.R: Added "mcmc density plots reject invalid bounds" test block

Coverage

Tests verify all validation branches via ppc_dens_overlay and mcmc_dens:

• Non-numeric bounds (c("a", "b"))
• Wrong length (c(0, 1, 2), 1)
• NA values (c(0, NA), c(NA, 1))
• Invalid ordering (c(1, 0), c(1, 1))

test_that("density PPC/PPD plots reject invalid bounds", {
  expect_error(ppc_dens_overlay(y, yrep, bounds = c("a", "b")),
               "`bounds` must be a numeric vector of length 2")
  expect_error(ppc_dens_overlay(y, yrep, bounds = c(1, 0)),
               "`bounds` must satisfy bounds\\[1\\] < bounds\\[2\\]")
})

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[VisruthSK]

Lgtm, dunno if the syntax for expect_error is correct though. Will see on test fails