Fix ArgumentNullException in Decoder.Convert and fix secure hash of ROS<byte>

[AArnott]

Fix ArgumentNullException thrown from Decoder.Convert

When a ReadOnlySequence<byte> that includes an empty ReadOnlyMemory<byte> segment is deserialized, the string interning converter may end up throwing ArgumentNullException.
This fixes that bug.

Fix secure hashing of ReadOnlySequence<byte>

Previously, it was producing multiple hashes for 'equal' sequences based on where the segment boundaries fell.
This change updates SipHash so that it properly supports streaming hashing such that the boundaries of each segment do not impact the resulting value.

[Copilot]

Pull request overview

This PR addresses two correctness issues in the serialization stack: (1) decoding/interning strings from fragmented ReadOnlySequence<byte> inputs that include empty segments, and (2) computing collision-resistant hashes over ReadOnlySequence<byte> values in a way that is independent of segment boundaries.

Changes:

• Add/extend tests to cover ReadOnlySequence<byte> inputs that include empty segments and to validate segment-boundary-independent hashing.
• Update Encoding.GetChars(ReadOnlySequence<byte>, ...) polyfill to safely handle empty sequences/segments during incremental decoding.
• Refactor SipHash to support incremental hashing over multi-segment sequences and update secure hashing call sites to use it.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/Nerdbank.MessagePack.Tests/StructuralEqualityComparerTests.cs	Expands equality/hash tests for ReadOnlySequence<byte> across segmented and empty-segment cases.
test/Nerdbank.MessagePack.Tests/StringInterningTests.cs	Adds coverage for fragmented sequences that include an empty segment during string interning.
test/Nerdbank.MessagePack.Tests/SequenceBuilder.cs	Introduces a test helper to build ReadOnlySequence<T> instances that retain empty segments.
test/Nerdbank.MessagePack.Tests/MessagePackReaderTests.ReadString.cs	Switches to SequenceBuilder and adds a reader test including an empty segment.
test/Nerdbank.MessagePack.Tests/MessagePackReaderTests.cs	Switches fragmented-sequence construction to SequenceBuilder.
test/Nerdbank.MessagePack.Tests/ArraysOfPrimitivesTests.cs	Adds a bool[] roundtrip test using a sequence with an empty segment.
src/Nerdbank.MessagePack/SecureHash/SipHash.cs	Adds incremental/streaming SipHash implementation and a ReadOnlySequence<byte> overload.
src/Nerdbank.MessagePack/SecureHash/HashCollisionResistantPrimitives.cs	Simplifies ReadOnlySequence<byte> secure hashing by delegating to SipHash.Compute(sequence).
src/Nerdbank.MessagePack/PolyfillExtensions.cs	Avoids Decoder.Convert calls with empty segments and handles empty sequences.
src/Nerdbank.MessagePack/Extension.cs	Updates secure hashing of Extension payloads to hash the ReadOnlySequence<byte> directly.