fix(server): multi step tool call (#13486)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- Bug Fixes
- Enforced a consistent step limit for AI responses across providers,
preventing excessively long generations in both text and streaming modes
for more predictable results.

- Refactor
- Centralized step limit configuration into a shared provider, ensuring
uniform behavior across providers and simplifying future maintenance.
- Standardized application of step limits in text generation and
streaming flows to align provider behavior and improve overall
reliability.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/providers/provider.ts

@@ -55,6 +55,7 @@ import {
 @Injectable()
 export abstract class CopilotProvider<C = any> {
   protected readonly logger = new Logger(this.constructor.name);
+  protected readonly MAX_STEPS = 20;
   protected onlineModelList: string[] = [];
   abstract readonly type: CopilotProviderType;
   abstract readonly models: CopilotProviderModel[];