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/shared/interfaces.ts

@@ -1,7 +1,10 @@
 import type {
+  FetchHistoryOptions,
+  FetchHistoryResult,
   LLMProvider,
   McpScope,
   McpTransport,
+  NormalizedMessage,
   ProviderMcpServer,
   UpsertProviderMcpServerInput,
 } from '@/shared/types.js';
@@ -30,11 +33,16 @@ export interface IProviderMcpRuntime {
   }>;
 }
 
-
 /**
- * Provider contract that both SDK and CLI families implement.
+ * Main provider contract for CLI and SDK integrations.
+ *
+ * Each concrete provider owns its MCP runtime plus the provider-specific logic
+ * for converting native events/history into the app's normalized message shape.
  */
 export interface IProvider {
   readonly id: LLMProvider;
   readonly mcp: IProviderMcpRuntime;
-}
+
+  normalizeMessage(raw: unknown, sessionId: string | null): NormalizedMessage[];
+  fetchHistory(sessionId: string, options?: FetchHistoryOptions): Promise<FetchHistoryResult>;
+}