fix(embedding): guard lazy dimension cache against concurrent first access - #2749

Fixes a data race in the lazy embedding dimension cache.

Problem

Dimensions() lazily embeds a probe string and caches the result in dims/dim with no synchronization:

func (e *embedder) Dimensions() int64 {
    if e.dim <= 0 {
        embeddings, _ := e.EmbedStrings(...)
        e.dim = int64(len(embeddings[0]))
    }
    return e.dim
}

When several retrieval/indexing goroutines first call Dimensions() concurrently, they race on the shared field (detected by go test -race) and issue duplicate embed requests.

Fix

Guard the lazy initialization with a sync.Mutex in both embedder (infra/embedding/impl/http/http.go) and denseOnlyWrap (infra/embedding/impl/wrap/dense_only.go). A failed probe is not cached, so transient embed failures can be retried on the next call.

Test

dimensions_test.go runs Dimensions() from 32 goroutines against an httptest mock. It passes with the fix and reports DATA RACE on the old code.

go test -race ./infra/embedding/impl/http/ -run TestDimensionsConcurrent