fix: off-by-one errors in code comment patches (#39887)

Code comments next to a diff hunk could show one line too high in the
conversation view vs. in the code view that showed them on the correct
line.

- Treat the hunk end in `CutDiffAroundLine` as exclusive, so the line
after a hunk falls back to the unchanged-line context
- Don't count `\ No newline at end of file` markers as lines when
cutting the hunk
- Share the unchanged-line fallback with migrated review comments

modules/git/diff_test.go

@@ -120,10 +120,18 @@ func TestCutDiffAroundLine(t *testing.T) {
 	assert.Empty(t, emptyResult)
 
 	// Line is out of scope
-	emptyResult, err = CutDiffAroundLine(strings.NewReader(exampleDiff), 434, false, 0)
+	emptyResult, err = CutDiffAroundLine(strings.NewReader(exampleDiff), 7, false, 3)
 	assert.NoError(t, err)
 	assert.Empty(t, emptyResult)
 
+	noNewlineDiff := "diff --git a/a b/a\n--- a/a\n+++ b/a\n@@ -1 +1 @@\n-a\n\\ No newline at end of file\n+a"
+	result, err = CutDiffAroundLine(strings.NewReader(noNewlineDiff), 1, false, 2)
+	assert.NoError(t, err)
+	assert.Equal(t, "diff --git a/a b/a\n--- a/a\n+++ b/a\n@@ -1,1 +1,1 @@\n-a\n\\ No newline at end of file\n+a", result)
+	result, err = CutDiffAroundLine(strings.NewReader(noNewlineDiff), 1, true, 2)
+	assert.NoError(t, err)
+	assert.Equal(t, "diff --git a/a b/a\n--- a/a\n+++ b/a\n@@ -1 +1 @@\n-a", result)
+
 	// Handle minus diffs properly
 	minusDiff, err := CutDiffAroundLine(strings.NewReader(breakingDiff), 2, false, 4)
 	assert.NoError(t, err)