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
 

app/models/vector_store/registry.rb

@@ -24,7 +24,14 @@ class VectorStore::Registry
       explicit = ENV["VECTOR_STORE_PROVIDER"].presence
       return explicit.to_sym if explicit && ADAPTERS.key?(explicit.to_sym)
 
-      # Default: use OpenAI when credentials are available
+      # Default routing:
+      #   - When the configured LLM provider is Anthropic (which has no hosted
+      #     vector store), fall back to the local pgvector adapter. The
+      #     Embeddable concern still pulls embeddings from EMBEDDING_URI_BASE /
+      #     OPENAI_ACCESS_TOKEN — Anthropic users typically point this at
+      #     Voyage AI, a local Ollama instance, or OpenAI embeddings.
+      #   - Otherwise, use OpenAI when credentials are available.
+      return :pgvector if Setting.llm_provider == "anthropic"
       :openai if openai_access_token.present?
     end
 

test/models/vector_store/registry_test.rb

@@ -50,6 +50,29 @@ class VectorStore::RegistryTest < ActiveSupport::TestCase
     end
   end
 
+  test "adapter_name defaults to pgvector when LLM_PROVIDER is anthropic" do
+    Setting.stubs(:llm_provider).returns("anthropic")
+    VectorStore::Registry.stubs(:openai_access_token).returns(nil)
+    ClimateControl.modify(VECTOR_STORE_PROVIDER: nil) do
+      assert_equal :pgvector, VectorStore::Registry.adapter_name
+    end
+  end
+
+  test "adapter_name routes anthropic installs to pgvector even when OpenAI key is present" do
+    Setting.stubs(:llm_provider).returns("anthropic")
+    VectorStore::Registry.stubs(:openai_access_token).returns("sk-test")
+    ClimateControl.modify(VECTOR_STORE_PROVIDER: nil) do
+      assert_equal :pgvector, VectorStore::Registry.adapter_name
+    end
+  end
+
+  test "explicit VECTOR_STORE_PROVIDER overrides anthropic default" do
+    Setting.stubs(:llm_provider).returns("anthropic")
+    ClimateControl.modify(VECTOR_STORE_PROVIDER: "qdrant") do
+      assert_equal :qdrant, VectorStore::Registry.adapter_name
+    end
+  end
+
   test "configured? delegates to adapter presence" do
     VectorStore::Registry.stubs(:adapter).returns(nil)
     assert_not VectorStore.configured?