Return range coordinates for review comments (#3193)

* Return range coordinates for review comments

* docs(pull-requests): describe review comment coordinates

Document nullable current and original range fields and assert their serialized omission behavior for single-line and outdated comments.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: yiheng-kkk <272397091+yiheng-kkk@users.noreply.github.com>
Co-authored-by: Sam Morrow <sammorrowdrums@github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>