Add focused coverage tests for plot and diagnostics

[ArslanBatyrov]

This PR adds two focused test files to increase coverage in EnvCpt:

• test-plot-extra.R for plot.envcpt() argument validation and behaviour
• test-diagnostics-extra.R for AIC / AICweights diagnostics behaviour

I ran the full local test suite with NOT_CRAN=true, and it completed with no failures.

I also compared package coverage on upstream main versus this branch using covr::package_coverage().

Coverage summary:

File	Before	After
Overall	87.02%	87.81%
R/diagnostics.R	80.08%	82.57%
src/C_cptreg.c	85.60%	85.60%
R/CptReg.R	94.35%	94.35%
R/envcpt.R	95.49%	95.49%

While adding diagnostics tests, I also noticed a likely separate bug in BIC.envcpt when n is supplied explicitly.

Reproducible example:

set.seed(1)
x <- c(rnorm(100, 0, 1), rnorm(100, 5, 1))
out <- envcpt(x)
BIC(out, n = length(x))