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/shared/base/abstract.provider.ts

@@ -1,8 +1,16 @@
 import type { IProvider, IProviderMcpRuntime } from '@/shared/interfaces.js';
-import type { LLMProvider } from '@/shared/types.js';
+import type {
+  FetchHistoryOptions,
+  FetchHistoryResult,
+  LLMProvider,
+  NormalizedMessage,
+} from '@/shared/types.js';
 
 /**
- * Shared MCP-only provider base.
+ * Shared provider base.
+ *
+ * Concrete providers must implement message normalization and history loading
+ * because both behaviors depend on each provider's native SDK/CLI event format.
  */
 export abstract class AbstractProvider implements IProvider {
   readonly id: LLMProvider;
@@ -11,4 +19,11 @@ export abstract class AbstractProvider implements IProvider {
   protected constructor(id: LLMProvider) {
     this.id = id;
   }
+
+  abstract normalizeMessage(raw: unknown, sessionId: string | null): NormalizedMessage[];
+
+  abstract fetchHistory(
+    sessionId: string,
+    options?: FetchHistoryOptions,
+  ): Promise<FetchHistoryResult>;
 }