Install-Uv piped the astral installer's entire output to Out-Null, so any
real failure (proxy block, AV quarantine, permissions) surfaced only as the
generic "uv installed but not found" message, and astral.sh was the sole
install source even though corporate proxies commonly block it while the
byte-identical GitHub releases installer downloads fine.
Three-rung ladder, all inside Install-Uv:
1. astral.sh installer with output captured via Tee-Object.
2. GitHub releases installer mirror (same UV_INSTALL_DIR).
3. Salvage an existing uv.exe (Get-Command uv, or the astral default
%USERPROFILE%\.local\bin\uv.exe) by copying it into $HermesHome\bin so
the managed-first invariant holds.
On total failure, print the last 15 lines of captured installer output plus
the existing manual-install pointer.
Reported by @BitBernd; proxy diagnosis by @gakugaku; Out-Null suppression
first identified by @webtecnica in #69366.
Closes#69216
3af56c2203