[GSoC 2026] M2.16: Fix part of #24717: Wired certificate assessment start and submission flow

[mosin74]

Overview

1. This PR fixes part of [Feature Request]: Learner certificates oppia/oppia#24717.
2. This PR does the following:
   • Auth guard: CertificateAssessmentPlayerPageAuthGuard now redirects to the 404 page both when the EnableCertificateAssessment feature flag is disabled and when the user is logged out, preventing logged-out users from reaching the player
   • Backend integration: Added API service methods startCertificateAssessmentAttemptAsync, submitCertificateAssessmentAttemptAsync, and getCertificateAssessmentQuestionAsync (plus new handler URL constants) and the CertificateAssessmentAttemptData model with typed backend.
   • Player root component: Fetches the real certificate offering on load and redirects to 404 when the certificate is unavailable; resolves the classroom URL fragment for banner exit buttons; manages intro/instructions/questions stages; handles start/submit failures gracefully (warning alerts).
   • Player page component: Replaced hardcoded mock questions with questions loaded from real question state; tracks answers per question id; derives correct answers from rule inputs (choice index for multiple-choice, content ids for multi-select); submits answers with question_id/is_correct/selected_answer.
   • Instruction panel / intro card: Updated to consume real offering data (title, description, time limit, total questions).
   • Spec updates: Full coverage for the auth guard, root component, player page component, instruction panel, intro card, backend API service, and model.

Essential Checklist

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• I have linked the issue that this PR fixes in the "Development" section of the sidebar.
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Proof that changes are correct

Screencast.from.2026-08-12.14-07-15.webm

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[mosin74]

@brianrodri Just a heads-up I've created the PR. I'll do one more pass and finalize the description, then I'll assign it to you. Thanks!