proton: detect incomplete default_pfx

[danyalahmed1995]

Summary

missing_default_prefix() only checked whether the default_pfx directory exists. If the directory exists but is empty or incomplete, Proton treats it as valid and skips rebuilding it with wineboot.

This updates the check to require a few expected default prefix files that later setup code depends on:

• system.reg
• drive_c/windows/system32/d3d8.dll
• drive_c/windows/syswow64/d3d8.dll

For #9660.

Testing

• Reproduced the validation failure with an empty/incomplete default_pfx directory.
• Verified an absent default_pfx is still treated as missing.
• Verified an empty default_pfx is now treated as missing.
• Verified a partially-created default_pfx without the expected sentinel files is treated as missing.
• Verified a sentinel-valid default_pfx is treated as present.
• Ran python -m py_compile proton steampipe_fixups.py.
• Ran git diff --check.