Make ReadOnlySequenceStream non-seekable to match net11

[SimonCropp]

ReadOnlySequenceStream shipped seekable in dotnet/runtime#126669, which is what the polyfill was written against. dotnet/runtime#132023 then made it intentionally non-seekable and removed its seek state and traversal logic, so all ten tests failed on the net11 leg while passing everywhere else.

Upstream rationale: backward positioning requires re-walking segments from the start, and ReadOnlySequence segment boundaries can be indirectly controlled by an untrusted network client through packet framing, so repeated seeks are worst case O(N) under adversarial fragmentation.

CanSeek now returns false, and Length, Position and Seek throw NotSupportedException. The long position field and the MoveTo rewind logic are gone; the SequencePosition cursor is the only state and only moves forward. ReadByte, CopyTo and CopyToAsync test remaining.IsEmpty instead of comparing against the sequence length.

Tests replace the six seek based cases with SeekingThrows (all six seek entry points) and ReadsAreForwardOnly (a failed seek must not disturb the cursor); position assertions elsewhere become drain checks.

Verified against the real net11 type with a reflection and behaviour probe. StringStream, ReadOnlyMemoryStream and WritableMemoryStream were checked the same way and already match: StringStream was never seekable, and the two memory streams stay seekable.