Fix ReadOnlySequence<T>.TryGet to return false at end position

[Copilot]

ReadOnlySequence<T>.TryGet(ref position, out memory) returned true with an empty memory when position was sequence.End (or sequence.GetPosition(size)). No segment is actually being returned in that case — it should return false.

var seq = new ReadOnlySequence<byte>(new byte[10]);
SequencePosition pos = seq.End;
bool ok = seq.TryGet(ref pos, out ReadOnlyMemory<byte> mem);
// before: ok == true,  mem.IsEmpty == true
// after:  ok == false, mem.IsEmpty == true

Changes

• ReadOnlySequence.Helpers.cs – TryGetBuffer now returns false with a default memory when the slice from position in the end segment would be empty (startIndex == endIndex). Covers both the multi-segment path (startSegment == endObject) and all single-segment paths (Array / String / MemoryManager<T>). Empty middle segments continue to return true + empty memory and advance, so multi-segment enumeration doesn't stall.
• ReadOnlySequence.cs – Added a <remarks> on TryGet documenting the end-of-sequence behavior.
• Tests – Updated tests that codified the previous behavior (Empty_TryGet, Empty_Enumerator, TryGetReturnsEmptySegments, TryGetStopsAtEndWhenEndIsFirstItemOfFull/Empty, EnumerableStopsAtEndWhenEndIsFirstItemOfFull/Empty) and added TryGetReturnsFalseAtEndPosition covering single-segment, empty, and multi-segment sequences with both sequence.End and sequence.GetPosition(size).

Behavior change

Public API semantic change. Loops of the form:

while (sequence.TryGet(ref position, out ReadOnlyMemory<T> memory)) { ... }

terminate one iteration earlier when the sequence has a trailing empty segment (or is empty) — an iteration that always yielded an empty memory. Iteration through empty middle segments is unchanged. SequenceReader<T> and the in-tree System.IO.Pipelines / System.Text.Json callers already tolerate TryGet returning false in this shape.

Note

This PR was authored by GitHub Copilot.