Fix Scala Native test binary resolution when cached metadata is present

[Copilot]

Scala Native test runs could pick .deder/out/<module>/nativeLink/metadata.json as the process to execute instead of the linked binary. In downstream builds, that surfaced as Permission denied when cached nativeLink outputs were reused.

• Root cause

  • ScalaNativeTasks selected the first matching file in the nativeLink output directory.
  • Cached tasks store metadata.json in the same directory, so file ordering could cause the JSON file to be treated as the executable.

• Resolution

  • Tighten native binary discovery in ScalaNativeTasks.findNativeBinary.
  • Exclude cache metadata and other non-binary artifacts from consideration.
  • Require an actual executable output instead of falling back to an arbitrary remaining file.

• Regression coverage

  • Add a focused unit test for the nativeLink output layout where both metadata.json and the native binary are present.
  • Assert that binary resolution selects the executable and ignores the cache file.

• Behavioral change

  • Failure mode is now explicit if no executable binary is present in nativeLink, rather than attempting to run a non-executable file.

private val ignoredFileSuffixes = Seq(".ll", ".c", ".o", ".s", ".json")

private[scalanative] def findNativeBinary(nativeLinkDir: os.Path): os.Path = {
  val files = os.list(nativeLinkDir).filter(os.isFile).sortBy(_.last)
  val candidates = files.filterNot(path => ignoredFileSuffixes.exists(path.last.endsWith))
  val executableCandidates = candidates.filter(isExecutableBinaryCandidate)

  executableCandidates.headOption.getOrElse(
    throw DederException(
      s"No executable native binary found in $nativeLinkDir. Files: ${files.map(_.last).mkString(", ")}"
    )
  )
}