refactor(providers): move session message delegation into sessions service

Move provider-backed session history and message normalization calls out of the
generic providers service so the service name reflects the behavior it owns.

Add a dedicated sessions service for listing session-capable providers,
normalizing live provider events, and fetching persisted session history through
the provider registry. Update realtime handlers and the unified messages route to
depend on `sessionsService` instead of `providersService`.

This separates session message operations from other provider concerns such as
auth and MCP, keeping the provider services easier to navigate as the module
grows.

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

@@ -7,17 +7,23 @@ import type {
 } from '@/shared/types.js';
 
 /**
- * Application service for provider message operations.
+ * Application service for provider-backed session 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 = {
+export const sessionsService = {
+  /**
+   * Lists provider ids that can load session history and normalize live messages.
+   */
   listProviderIds(): LLMProvider[] {
     return providerRegistry.listProviders().map((provider) => provider.id);
   },
 
+  /**
+   * Normalizes one provider-native event into frontend session message events.
+   */
   normalizeMessage(
     providerName: string,
     raw: unknown,
@@ -26,6 +32,9 @@ export const providersService = {
     return providerRegistry.resolveProvider(providerName).normalizeMessage(raw, sessionId);
   },
 
+  /**
+   * Fetches normalized persisted session history for one provider/session pair.
+   */
   fetchHistory(
     providerName: string,
     sessionId: string,