Adds TestTranslate_HunkHeaderAtEnd covering the edge case where a
hunk-header is the last position in the file with no subsequent
new-file line. Mirrors TestBuildPositionToLineMap_DeletionAtEnd for
the hunk-header code path.
Addresses NIT from sonnet-review-bot on PR #104 (comment 18412).
BuildPositionToLineMap incremented position and updated maxPositions for
@@ hunk-header lines but did not store a map entry, causing Translate()
to return a hard error for any comment positioned at a hunk header.
Store sentinel value 0 for hunk-header positions (analogous to -1 for
deletions) and extend Translate() to fall through to the nearest
context/addition line below, matching the existing deletion-line
behavior.
Fixes#97
claw