Handle DocFX exit codes in BuildCommand

[Copilot]

DocFX failures were silent because RunCommand didn't return the process exit code. A non-zero exit meant the caller had no way to detect failure without checking file existence afterward—a weak, delayed signal.

Changes

• Utils.RunCommand signature: Now returns Task<int> instead of Task to surface the exit code
• BuildCommand: Captures the exit code and immediately checks for failure:

  int exitCode = await Utils.RunCommand("docfx", docFxArguments, ...);
  
  if (exitCode != 0)
  {
      result = false;
      logger.LogError("DocFX exited with code {ExitCode} for Unity '{RepositoryTag}'", exitCode, repositoryTag);
      continue;
  }

This makes DocFX errors visible at the point of failure rather than relying on downstream checks. The version is now skipped immediately with a clear error log.

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Note: This extracts a focused fix from a larger PR to isolate exit code handling from unrelated changes (regex and pagination fixes).