fix(tutor): propagate request cancellation

[ttbombadil]

Summary

• Propagate browser and HTTP disconnect cancellation through tutor routes, course resolution, TutorService, provider model-list/completion requests, and content fetches.
• Avoid session progression commits after an aborted request.
• Keep coalesced Course Content loads alive for remaining waiters, return promptly to canceled waiters, and stop the shared load when its final waiter leaves.
• Preserve successful Tutor behavior and use only fake providers in tests.

Verification

• Full unit suite: 2,760 passed, 1 skipped.
• Tutor/Course Content quality suite: 416 passed.
• npm run check and relevant ESLint passed.
• Pre-push full unit, coverage validation, and Sonar quality gate passed.
• No live provider calls were made.