Fix TextReader.ReadAsync(Memory<char>) dropping data for non array-backed memory

[SimonCropp]

Bug

TextReader.ReadAsync(Memory<char>, CancellationToken) fell back to buffer.ToArray() when the memory was not array-backed (MemoryMarshal.TryGetArray returns false, e.g. a custom MemoryManager<char> or native memory). The read went into that temporary copy and was never copied back, so the caller's buffer stayed untouched while the returned count said chars had been read.

Fix

Same approach as the BCL: rent an array from ArrayPool<char>.Shared, read into it, copy the chars read into the destination memory, and return the array. The array is only returned to the pool once the underlying read has completed, since a cancellation via WaitAsync can leave the read in flight. The array-backed path is unchanged.

Test

TextReader_ReadAsync_Memory_NotArrayBacked reads into a slice of a MemoryManager<char> that does not override TryGetArray and checks the destination contents. Fails on net48 before the fix; passes on net48 and on net10.0 (BCL).