bug(multimodal): POST /search/cross-modal turns an embedding refusal into an empty 200 #18269

Defect

autobot-backend/api/multimodal.py cross_modal_search catches Exception and returns CrossModalSearchResponse(results={}, total_found=0). After PR #18230, NPUSemanticSearch.cross_modal_search raises EmbeddingUnavailableError, because none of the accelerator collections has a recorded embedding space (#18217). The route therefore reports an engine refusal as "no matches", which is the silent degradation #18185 forbids.

This was not fixed in #18230: open PR #18236 owns api/multimodal.py (same-file rule).

Acceptance criteria

• The route answers an EmbeddingUnavailableError with a 503 and its reason, using api.embedding_refusal.reraise_http_or_refusal from fix(knowledge): one active embedding model resolver, model-aware cache keys, and search refuses rather than embed in another space (#18143, #18185) #18230.
• A test goes through the route.

Refs #18185, #18217, #18230.