  Transmission spectroscopy is enabling precise measurements of atmospheric H2O
abundances for numerous giant exoplanets. For hot Jupiters, relating H2O
abundances to metallicities provides a powerful probe of their formation
conditions. However, metallicity measurements for Neptune-mass exoplanets are
only now becoming viable. Exo-Neptunes are expected to possess super-solar
metallicities from accretion of H2O-rich and solid-rich planetesimals. However,
initial investigations into the exo-Neptune HAT-P-26b suggested a significantly
lower metallicity than predicted by the core-accretion theory of planetary
formation and solar system expectations from Uranus and Neptune. Here, we
report an extensive atmospheric retrieval analysis of HAT-P-26b, combining all
available observations, to reveal its composition, temperature structure, and
cloud properties. Our analysis reveals an atmosphere containing
1.5(+2.1)(-0.9)% H2O, an O/H of 18.1(+25.9)(-11.3)x solar, and C/O < 0.33 (to
2$\sigma$). This updated metallicity, the most precise exo-Neptune metallicity
reported to date, suggests a formation history with significant planetesimal
accretion, albeit below that of Uranus and Neptune. We additionally report
evidence for metal hydrides at 4.1$\sigma$ confidence. Potential candidates are
identified as TiH (3.6$\sigma$), CrH (2.1$\sigma$), or ScH (1.8$\sigma$).
Maintaining gas-phase metal hydrides at the derived temperature (~560 K)
necessitates strong disequilibrium processes or external replenishment.
Finally, we simulate the JWST Guaranteed Time Observations for HAT-P-26b.
Assuming a composition consistent with current observations, we predict JWST
can detect H2O (at 29$\sigma$), CH4 (6.2$\sigma$), CO2 (13$\sigma$), and CO
(3.7$\sigma$), improving metallicity and C/O precision to 0.2 dex and 0.35 dex.
Furthermore, NIRISS observations could detect several metal hydrides at
>5$\sigma$ confidence.
