feat(ai): default Anthropic installs to pgvector RAG (4/5)

The provider-agnostic vector store stack (VectorStore::Pgvector + the
Embeddable concern) already shipped to main. This PR closes the
Anthropic loop:

- VectorStore::Registry.adapter_name now returns :pgvector when
  Setting.llm_provider == "anthropic" and no explicit
  VECTOR_STORE_PROVIDER override is set. Anthropic has no hosted vector
  store, so falling back to the local pgvector adapter is the only
  correct default. Explicit VECTOR_STORE_PROVIDER still wins.
- SearchFamilyFiles surfaces a longer message when no adapter is wired
  up — calling out pgvector + EMBEDDING_URI_BASE as the supported
  Anthropic-only path so the user is not stuck with an "OpenAI required"
  hint that is no longer accurate.

The Embeddable concern already pulls embeddings from
EMBEDDING_URI_BASE / EMBEDDING_ACCESS_TOKEN (with OpenAI as fallback),
so Anthropic installs point this at Voyage AI, a local Ollama instance,
or OpenAI embeddings — independent of the chat provider.

Tests cover the new default routing, the existing OpenAI default
staying intact, and explicit VECTOR_STORE_PROVIDER overriding the
Anthropic default.

Stacked on #1985 (PR 3/5). 5/5 settings UI + retention disclosure next.

app/models/assistant/function/search_family_files.rb

@@ -71,7 +71,10 @@ class Assistant::Function::SearchFamilyFiles < Assistant::Function
       return {
         success: false,
         error: "provider_not_configured",
-        message: "No vector store is configured. Set VECTOR_STORE_PROVIDER or configure OpenAI."
+        message: "No vector store is configured. Set VECTOR_STORE_PROVIDER " \
+                 "(openai | pgvector | qdrant), configure OpenAI, or — for " \
+                 "Anthropic-only installs — enable the pgvector adapter and " \
+                 "point EMBEDDING_URI_BASE at an embeddings endpoint."
       }
     end