feat(ai): honor Setting.llm_provider for batch and PDF flows (#2265)

Auto-categorization, merchant detection/enhancement, and PDF/bank-statement
extraction hard-coded Provider::Registry.get_provider(:openai), so selecting
Anthropic (or running an Anthropic-only self-hosted install) left those
operations using/missing OpenAI rather than the chosen provider.

Add Provider::Registry.preferred_llm_provider, which resolves the LLM provider
honoring Setting.llm_provider with a configured-provider fallback (mirroring how
chat picks its provider), and route all six TODO(#2113) call sites through it:

- Family::AutoCategorizer#llm_provider
- Family::AutoMerchantDetector#llm_provider
- ProviderMerchant::Enhancer#llm_provider
- PdfImport (process_pdf + extract_bank_statement)
- Assistant::Function::ImportBankStatement

Provider::Anthropic already implements auto_categorize / auto_detect_merchants /
enhance_provider_merchants (#1984) and process_pdf / extract_bank_statement
(#1985), so no provider changes are needed — only the wiring.

Closes #2113.

app/models/assistant/function/import_bank_statement.rb

@@ -92,15 +92,15 @@ class Assistant::Function::ImportBankStatement < Assistant::Function
       }
     end
 
-    # Extract transactions from the PDF using provider
-    # TODO(#2113): hardcoded to OpenAI. Provider::Anthropic implements
-    # extract_bank_statement (PR #1985); this should honor Setting.llm_provider.
-    provider = Provider::Registry.get_provider(:openai)
+    # Extract transactions from the PDF using the configured LLM provider.
+    # Honors Setting.llm_provider (issue #2113) — Provider::Anthropic implements
+    # extract_bank_statement (PR #1985).
+    provider = Provider::Registry.preferred_llm_provider
     unless provider
       return {
         success: false,
         error: "provider_not_configured",
-        message: "OpenAI provider is not configured"
+        message: "AI provider is not configured"
       }
     end