fix(computer): preserve activation and observation result contracts (…

fix(computer): preserve activation and observation result contracts (#143953)

Report verified CUA window activation as success even when the SDK exposes
a diagnostic code. Preserve genuine error and structured refusal handling.

Reject targeted desktop screenshot and wait requests before capture, and
guide agents toward window observations with their own observation IDs.
Clarify the desktop/window distinction in tool guidance and documentation.

Validation: original failures reproduced; 61 focused tests, complete
changed-file checks, and managed P0-P2 review passed.