System.Reflection.Metadata: preserve pre-linked suffix chain when linking into empty BlobBuilder

[Copilot]

BlobBuilder.LinkSuffix could drop already-linked content when the destination builder was empty and the incoming suffix had its own linked suffix chain. This change preserves the full chain so byte order and total content are retained (11-22, not 22-00).

• Behavior fix (BlobBuilder.LinkSuffix)

  • Added an explicit empty-destination path after head-buffer swap.
  • When destination is empty, destination now adopts the suffix chain tail (_nextOrPrevious) instead of leaving chain links in a state that loses prior suffix content.
  • Non-empty destination relinking logic is unchanged.

• Regression coverage (BlobTests)

  • Added LinkSuffix_EmptyDestination_SuffixAlreadyLinked.
  • Repro shape:
    • element contains 0x11
    • tail contains 0x22
    • element.LinkSuffix(tail)
    • emptyPrefix.LinkSuffix(element)
  • Asserts resulting bytes are { 0x11, 0x22 } and counts remain consistent across builders.

element.LinkSuffix(tail);
emptyPrefix.LinkSuffix(element);

AssertEx.Equal(new byte[] { 0x11, 0x22 }, emptyPrefix.ToArray());

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[Copilot]

Pull request overview

Fixes BlobBuilder.LinkSuffix to correctly preserve an incoming suffix’s existing linked-suffix chain when linking into an empty destination builder, ensuring previously-linked content isn’t dropped and byte order/content are retained.

Changes:

• Update BlobBuilder.LinkSuffix to adopt the suffix chain tail (_nextOrPrevious) when the destination builder is empty.
• Add a regression test covering “empty destination + suffix already linked” to verify byte sequence and counts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/BlobBuilder.cs	Adjusts empty-destination LinkSuffix relinking to preserve an already-linked suffix chain.
src/libraries/System.Reflection.Metadata/tests/Metadata/BlobTests.cs	Adds regression coverage for linking a pre-linked suffix into an empty destination.