[hardening] Add OpenXmlMarkupService unit tests (comments + tracked changes fidelity)

[MTCMarkFranco]

Enhancement

LambdaRag.Markup (the most user-visible output module) has no direct unit tests. Existing coverage:

• Markup/AnnotationFactoryPassTests.cs — exercises annotation construction, not docx output.
• Markup/CommentFormattingTests.cs — exercises text-level comment formatting only.
• tests/.../ReviewedDocxIdempotency.cs — verifies byte-stability but not correctness of comment / ins / del placement.

Goal: A focused OpenXmlMarkupServiceTests suite that loads a small fixture docx, applies a known annotation set, opens the output with OpenXml, and asserts:

• Comment count and IDs match expected.
• Comment.Author, Comment.Initials, Comment.Date set to expected (deterministic) values.
• CommentRangeStart / CommentRangeEnd brackets enclose the expected run text.
• InsertedRun text matches Annotation.Replacement.
• DeletedRun text matches the original span text (depends on [hardening] DeletedRun writes literal '[deleted]' instead of original span text #53).
• Output is byte-identical across two runs (idempotency, narrower fixture than the existing test).

Out of scope

• End-to-end pipeline tests (already covered in integration suite).