* feat(providers): add Mixedbread AI as embeddings provider (#6660)
Registers Mixedbread AI (https://api.mixedbread.com) in the
EMBEDDING_PROVIDERS registry alongside the other bearer-auth embedding
providers (Voyage AI, Jina AI, Nomic, ...): OpenAI-compatible
/v1/embeddings endpoint, exposing mxbai-embed-large-v1 and
mxbai-embed-2d-large-v1 (both 1024d, Matryoshka). Adds a matching
provider metadata entry (icon/color/authHint/free-tier note) modeled
on the nomic block, regenerates docs/reference/PROVIDER_REFERENCE.md,
and syncs the 250->251 provider-count mentions in README/AGENTS/CLAUDE
required by the strict docs-counts gate.
No executor/translator changes needed — the embeddings handler is a
generic pass-through with no provider-specific branching.
* test(providers): align APIKEY_PROVIDERS count 167→168 for the new 6660 provider (#6660)
Adding the mixedbread embeddings provider to specialty-media.ts grows APIKEY_PROVIDERS
by one; providers-constants-split.test.ts hardcodes the family-partition
total. Legitimate count alignment (the code genuinely added a provider),
not a weakened assertion — all 4 partition/dedup checks still enforced.
