fix(slack): don't double-emit trailing text on odd number of ``` fences (#1436)

extractCodeBlocks split on "```" and treated every odd-indexed part as a
closed code block. With an odd number of fences (>=3), Split returns an even
number of parts, so the final segment -- which follows an unpaired opening
fence and must stay literal -- landed on an odd index and was both emitted as
a \x00CB\x00 placeholder (later restored as ```seg```) AND re-appended
verbatim by the trailing len(parts)%2==0 block. The segment was therefore
duplicated in the rendered Slack message (and blocks gained a spurious entry).
Unclosed trailing fences are common in streamed LLM output.

Exclude the final unpaired part from the paired-scan loop. Add a regression
test covering the closed-block-then-unpaired-fence case.

internal/channels/slack/format.go

@@ -113,8 +113,17 @@ func extractCodeBlocks(text string) (blocks []string, result string) {
 		return nil, text
 	}
 
+	// An odd number of ``` fences makes Split return an even number of parts;
+	// the final segment then follows an unpaired opening fence and must stay
+	// literal (it is re-appended verbatim below), so exclude it from the
+	// paired-scan loop instead of misclassifying it as a closed code block.
+	scanEnd := len(parts)
+	if scanEnd%2 == 0 {
+		scanEnd--
+	}
 	var sb strings.Builder
-	for i, part := range parts {
+	for i := 0; i < scanEnd; i++ {
+		part := parts[i]
 		if i%2 == 1 {
 			blocks = append(blocks, part)
 			sb.WriteString(fmt.Sprintf("\x00CB%d\x00", len(blocks)-1))

internal/channels/slack/format_test.go

@@ -318,6 +318,14 @@ func TestExtractCodeBlocks(t *testing.T) {
 			input:         "text\n```\ncode (no closing)",
 			expectedCount: 0,
 		},
+		{
+			// A closed block followed by an unpaired opening fence: 3 fences ->
+			// 4 parts. Only the middle part is a real code block; the trailing
+			// segment must stay literal, not be counted (and re-emitted) as one.
+			name:          "closed block then unpaired trailing fence",
+			input:         "a```b```c```d",
+			expectedCount: 1,
+		},
 		{
 			name:          "empty code block",
 			input:         "```\n\n```",