Replace CachedFragment::Copy with std::move in CacheTsbFragment

[benmesander]

Eliminate unnecessary deep copy of fragment data (2-7 MB for UHD) in CacheTsbFragment by using move assignment instead of Copy(). The move assignment operator already exists on CachedFragment.

Also add null guard for GetFetchChunkBuffer return value to prevent potential null pointer dereference. (bug discovered while doing above)

LL-DASH chunk mode callers that need the fragment for both CacheTsbFragment and EnqueueWrite now create a separate copy before moving into the cache.

Add unit test validating move semantics transfer ownership correctly.

[DomSyna]

same as above

[Copilot]

Pull request overview

This PR optimizes TSB fragment caching by replacing a deep copy of CachedFragment payloads with move assignment, reducing large (multi-MB) memory copies in UHD scenarios, and adds a null guard to prevent a potential null dereference when acquiring a chunk-cache slot.

Changes:

• Update MediaStreamContext::CacheTsbFragment to move-assign fragment data into the cache slot, avoiding deep copies.
• Add a null check for GetFetchChunkBuffer(true) returning nullptr, returning false early with an error log.
• Update LL-DASH chunk-mode paths to create a separate copy when the fragment must be used for both chunk caching and EnqueueWrite, and add a unit test intended to validate move semantics.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
MediaStreamContext.cpp	Replace Copy() with move assignment in CacheTsbFragment, add null guard, and adjust LL-DASH callers to copy before caching.
test/utests/tests/MediaStreamContextTests/FragmentDownloadTests.cpp	Add a unit test for CacheTsbFragment move-based transfer into the chunk-cache slot.