fix(DIS-105): prevent fence closure from pushing chunks beyond 2000-char hard limit

The flush() helper now enforces the Discord hard limit by hard-splitting
any chunk that exceeds 2000 chars (e.g. when "\n```" was appended after
the soft-limit check). Also reserves 4 chars of overhead in the
soft-limit check when inside a fenced code block.

Adds a regression test that verifies no chunk exceeds 2000 chars even
when the fence closure is appended at the limit boundary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/discord/split.ts

@@ -28,6 +28,12 @@ export function splitForDiscord(text: string, limit = 1900): string[] {
   let fenceLang = "";
 
   const flush = () => {
+    // Hard-split if current exceeds the Discord hard limit (e.g. because
+    // a fence closure "\n```" was appended after the soft-limit check).
+    while (current.length > DISCORD_HARD_LIMIT) {
+      chunks.push(current.slice(0, DISCORD_HARD_LIMIT));
+      current = current.slice(DISCORD_HARD_LIMIT);
+    }
     if (current.length > 0) {
       chunks.push(current);
       current = "";
@@ -43,7 +49,10 @@ export function splitForDiscord(text: string, limit = 1900): string[] {
     const separator = current.length > 0 ? "\n" : "";
     const addition = separator + line;
 
-    if (current.length + addition.length > effectiveLimit) {
+    // Reserve space for a closing fence (``` + newline) when inside a code block
+    const fenceOverhead = inFence ? 4 : 0; // "\n```".length
+
+    if (current.length + addition.length + fenceOverhead > effectiveLimit) {
       // Need to flush before adding this line
       if (inFence) {
         // Close the open fence before flushing

tests/unit/split-for-discord.test.ts

@@ -101,4 +101,20 @@ describe("splitForDiscord", () => {
     const idx = result.indexOf(closingChunk!);
     expect(result[idx + 1]).toMatch(/^```python/);
   });
+
+  // Regression: fence closure "\n```" must not push a chunk beyond the 2000 hard limit
+  it("does not exceed 2000 chars even when fence closure is appended at limit boundary", () => {
+    const fenceOpen = "```typescript\n";
+    // Line long enough that current is near 2000 when fence closure is added
+    const longLine = "x".repeat(1984);
+    const nextLine = "y".repeat(10);
+    const fenceClose = "\n```";
+
+    const text = fenceOpen + longLine + "\n" + nextLine + fenceClose;
+    const result = splitForDiscord(text, 2000);
+
+    for (const chunk of result) {
+      expect(chunk.length).toBeLessThanOrEqual(DISCORD_HARD_LIMIT);
+    }
+  });
 });