refactor(providers): centralize message handling in provider module

Move provider-specific normalizeMessage and fetchHistory logic out of the legacy
server/providers adapters and into the refactored provider classes so callers can
depend on the main provider contract instead of parallel adapter plumbing.

Add a providers service to resolve concrete providers through the registry and
delegate message normalization/history loading from realtime handlers and the
unified messages route. Add shared TypeScript message/history types and normalized
message helpers so provider implementations and callers use the same contract.

Remove the old adapter registry/files now that Claude, Codex, Cursor, and Gemini
implement the required behavior directly.

server/modules/providers/services/providers.service.ts

@@ -0,0 +1,36 @@
+import { providerRegistry } from '@/modules/providers/provider.registry.js';
+import type {
+  FetchHistoryOptions,
+  FetchHistoryResult,
+  LLMProvider,
+  NormalizedMessage,
+} from '@/shared/types.js';
+
+/**
+ * Application service for provider message operations.
+ *
+ * Callers pass a provider id and this service resolves the concrete provider
+ * class, keeping normalization/history call sites decoupled from implementation
+ * file layout.
+ */
+export const providersService = {
+  listProviderIds(): LLMProvider[] {
+    return providerRegistry.listProviders().map((provider) => provider.id);
+  },
+
+  normalizeMessage(
+    providerName: string,
+    raw: unknown,
+    sessionId: string | null,
+  ): NormalizedMessage[] {
+    return providerRegistry.resolveProvider(providerName).normalizeMessage(raw, sessionId);
+  },
+
+  fetchHistory(
+    providerName: string,
+    sessionId: string,
+    options?: FetchHistoryOptions,
+  ): Promise<FetchHistoryResult> {
+    return providerRegistry.resolveProvider(providerName).fetchHistory(sessionId, options);
+  },
+};