revert: computer-use cua-driver (PR #16919) (#16927)

Reverts PR #16919 (commits dad10a78d, 413ee1a28, b4a8031b2, afb958829)
which was merged prematurely. Restoring the pre-merge state so #14817
and #15328 can be revisited as standing PRs.

Reverted commits:
- afb958829 fix(computer-use): harden image-rejection fallback + AUTHOR_MAP
- b4a8031b2 fix(computer-use): unwrap _multimodal tool results
- 413ee1a28 feat(computer-use): background focus-safe backend
- dad10a78d feat(computer-use): cua-driver backend, universal any-model schema

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

agent/display.py

@@ -827,10 +827,6 @@ def _detect_tool_failure(tool_name: str, result: str | None) -> tuple[bool, str]
                 return True, " [full]"
 
     # Generic heuristic for non-terminal tools
-    # Multimodal tool results (dicts with _multimodal=True) are not strings —
-    # treat them as successes since failures would be JSON-encoded strings.
-    if not isinstance(result, str):
-        return False, ""
     lower = result[:500].lower()
     if '"error"' in lower or '"failed"' in lower or result.startswith("Error"):
         return True, " [error]"