Replace the 'describe everything in thorough detail' auto image-preprocess
prompt with a concise 2-4 sentence summary prompt so image-bearing gateway
messages stop generating ~2000-char descriptions (35s+ on local models).
Prompt-only variant of #10852: the max_tokens=500 cap and the
preserve_max_tokens aux-client plumbing from the original PR are
intentionally dropped to stay compatible with the max-tokens-knob policy
direction (#75253 removes hardcoded vision caps).
Fixes#10809
gnanirahulnutakki