The `dimensions` request param truncated AFTER normalizing, so truncated
vectors were no longer unit-norm and downstream cosine-via-dot-product math
broke. Truncate then re-normalize (OpenAI semantics), shared by all backends.
Verified on MRL-trained Qwen3-Embedding-4B: at 1024/2560 dims retrieval quality
is unchanged (0.900 vs 0.181 relevant/unrelated; full-dim was 0.898 vs 0.189).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EPLnsRpNBzWCHLgkXATqRz