Perf: Refactor synchronous ValueTask waits in Videodec2Decoder to avoid allocations

This avoids unconditionally calling `.AsTask().GetAwaiter().GetResult()` on `ValueTask` from Channel.Reader/Writer. Instead, we use `.IsCompletedSuccessfully` to fast-path successful synchronous completions, completely bypassing the allocation of a backing `Task` object.

This is a significant optimization for Videodec2Decoder, which relies on channel reads and writes inside tight worker and scheduler loops.

Also fixes REUSE compliance by removing unnecessary test files, and fixes a few CI warnings and missing symbols in ps5_names.txt.

Co-authored-by: manupawickramasinghe <73810867+manupawickramasinghe@users.noreply.github.com>

Author: google-labs-jules[bot]