feat(social): bound challenge submission attempts (BA-057)

[ranjeetprabhatedu-dotcom]

Summary

This PR addresses issue #970 (BA-057) by enforcing a bounded retry/attempt limit on challenge submissions with boundary test coverage before, at, and beyond the configured limit.

• Changes:
  • BackendAcademy/src/social/dto/challenge.dto.ts: Added optional maxAttempts?: number configuration to CreateChallengeDto with @IsOptional(), @IsInt(), and @Min(1).
  • BackendAcademy/src/social/challenge.service.ts: Enforce attempt tracking and retry limits per user per challenge (defaulting to 3 attempts), rejecting excessive submissions with BadRequestException.
  • BackendAcademy/src/social/challenge.service.spec.ts: Added comprehensive unit tests asserting behavior before the limit, at the limit, beyond the limit, custom configured limits, and input validation.
• Verification:
  • All 27 tests in challenge.service.spec.ts pass cleanly (100% Green).
  • Scope strictly restricted to the 3 allowlisted files.

Closes #970