fix: validate AI agent request text correctly

src/ai/dto/create-agent-plan.dto.ts

@@ -1,5 +1,5 @@
 import { Type } from 'class-transformer';
-import { ArrayMaxSize, IsArray, IsInt, IsOptional, IsString, IsUUID, Max, MaxLength, Min, ValidateNested } from 'class-validator';
+import { ArrayMaxSize, IsArray, IsInt, IsOptional, IsString, IsUUID, Max, MaxLength, Min, MinLength, ValidateNested } from 'class-validator';
 
 class AgentWorkspaceFileDto {
   @IsString()
@@ -39,7 +39,7 @@ export class CreateAgentPlanDto {
   week?: number;
 
   @IsString()
-  @Min(1)
+  @MinLength(1)
   @MaxLength(4000)
   request: string;