  We present models designed to quantify the effects of stellar activity on
exoplanet transit spectroscopy and atmospheric characterization at low (R =
100) and high (R = 100,000) spectral resolution. We study three model classes
mirroring planetary system archetypes: a hot Jupiter around an early-K star (HD
189733 b); a mini-Neptune around an early-M dwarf (K2-18 b); and terrestrial
planets around a late M dwarf (TRAPPIST-1). We map photospheres with
temperatures and radial velocities (RV) and integrate specific intensity
stellar models. We obtain transit spectra affected by stellar contamination,
the Rossiter--McLaughlin effect (RME), and center-to-limb variations (CLV). We
find that, at low resolution, for later-type stars, planetary water features
become difficult to distinguish from contamination. Many distributions of
unocculted active regions can induce planetary-like features of similar
amplitudes in the case of a late M dwarf. Atmospheric characterization of
planets around late-type stars will likely continue to suffer from degeneracy
with stellar activity unless active regions' parameters can be constrained
using additional information. For the early-K star, stellar contamination
mostly manifests itself through a slope at optical wavelengths similar to
Rayleigh scattering. In all cases, contamination induces offsets in measured
planet radii. At high resolution, we show that we can determine the origin of
$\text{H}_2$O and CO detection signals and lift the degeneracy observed at low
resolution, provided sufficient planet RV variation during transit and adequate
correction for the RME and CLV when required. High-resolution spectroscopy may
therefore help resolve issues arising from stellar contamination for favorable
systems.
