  We present a new optical transmission spectrum of the hot Jupiter WASP-79b.
We observed three transits with the STIS instrument mounted on HST, spanning
0.3 - 1.0 um. Combining these transits with previous observations, we construct
a complete 0.3 - 5.0 um transmission spectrum of WASP-79b. Both HST and
ground-based observations show decreasing transit depths towards blue
wavelengths, contrary to expectations from Rayleigh scattering or hazes. We
infer atmospheric and stellar properties from the full near-UV to infrared
transmission spectrum of WASP-79b using three independent retrieval codes, all
of which yield consistent results. Our retrievals confirm previous detections
of H$_{2}$O (at 4.0$\sigma$ confidence), while providing moderate evidence of
H$^{-}$ bound-free opacity (3.3$\sigma$) and strong evidence of stellar
contamination from unocculted faculae (4.7$\sigma$). The retrieved H$_{2}$O
abundance ($\sim$ 1$\%$) suggests a super-stellar atmospheric metallicity,
though stellar or sub-stellar abundances remain consistent with present
observations (O/H = 0.3 - 34$\times$ stellar). All three retrieval codes obtain
a precise H$^{-}$ abundance constraint: log(X$_{\rm{H^{-}}}$) $\approx$ -8.0
$\pm$ 0.7. The potential presence of H$^{-}$ suggests that JWST observations
may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred
faculae are $\sim$ 500 K hotter than the stellar photosphere, covering $\sim$
15$\%$ of the stellar surface. Our analysis underscores the importance of
observing UV - optical transmission spectra in order to disentangle the
influence of unocculted stellar heterogeneities from planetary transmission
spectra.
