The download endpoint had a dangerous fallback that silently served the
wrong binary when the requested platform/arch combination was missing.
If a Docker image shipped without Windows binaries, the installer would
receive a Linux ELF instead of a Windows PE, causing ERROR_BAD_EXE_FORMAT.
Changes:
- Download handler now operates in strict mode when platform+arch are
specified, returning 404 instead of serving mismatched binaries
- PowerShell installer validates PE header (MZ signature)
- PowerShell installer verifies PE machine type matches requested arch
- PowerShell installer fetches and verifies SHA256 checksums
- PowerShell installer shows diagnostic info: OS arch, download URL,
file size for better troubleshooting
This prevents silent failures and provides clear error messages when
binaries are missing or corrupted.
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