fix: refresh Android course structure when requested

[edschema]

Description

Fixes Android course-structure refresh requests that could return the first cached Flow emission instead of requesting an updated structure.

When getCourseStructure(courseId, isNeedRefresh = true) runs online, it now uses a dedicated request path with Cache-Control: no-cache. Offline refreshes remain cache-only, and ordinary non-refresh callers keep the existing cache-first behavior.

Out of Scope

This is the Android prerequisite for later course-routing work. It does not add Inbox or routing behavior, redesign sibling status, Dates, progress, or enrollment caches, change production callers, or modify server contracts, Room schema, or migrations.

Change Log

• Add a separate coalesced fresh course-structure request path.
• Preserve cache-first Flow behavior for non-refresh and offline requests.
• Prevent delayed requests from an older course session from replacing the newer session final state.
• Add focused coverage for headers, request coalescing, persistence ordering, session reset, Room races, and refresh markers.

Notes

Fresh and cache-first requests intentionally use separate coalescers so a refresh cannot join a request using the cache-first header. The public CourseInteractor interface and CourseApi contract are unchanged.

Tests Done

• Focused CoalescingCacheTest, CourseRepositoryFreshTest, and CourseInteractorFreshTest
• :course:testDevelopDebugUnitTest
• :app:assembleDevelopDebug
• detektAll
• Pixel checks for online refresh, offline cached use, and a controlled changed course-structure response
• Android lint ran; it reports only established baseline findings outside this change

AI Assistance

AI tools were used to aid in diagnosis, planning, implementation, testing, and review. All work was checked and reviewed.

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[edschema]

Closing to make some changes. Will open new pr afterwards