fix: don't emit empty agent_message_chunk (#841)

## Why this is needed

`toAcpNotifications` can emit an `agent_message_chunk` whose content is
an empty string. This happens in two situations:

1. **Streamed text deltas** — the `text` / `text_delta` case builds the
update unconditionally from `chunk.text`, and the Claude Agent SDK can
stream zero-length text deltas (for example, an empty trailing assistant
block at the end of a turn).
2. **Full-string content** — the string-content path emits a chunk even
when the string is empty.

An empty message chunk carries no information, so at best it is noise.
At worst it breaks clients: the ACP spec does not forbid empty content,
so clients handle it inconsistently and some throw. In JetBrains Air the
frontend event consumer throws on an empty-content `agent_message_chunk`
(`Failed to handle agent event`); once that happens the transcript is
**frozen for the rest of the turn** — no further updates render and
queued prompts are never dispatched. In practice this reproduced
repeatedly around image reads and end-of-turn, and the only recovery was
restarting the IDE.

Because the adapter is client-agnostic, this affects any ACP client that
is strict about chunk content, not a single editor — so the correct
place to fix it is here, in the adapter, rather than working around it
in each client.

## Fix

Do not emit empty content. Both call sites are guarded, mirroring the
empty-`thinking` guard that already exists a few lines below in the same
function, so behavior is consistent across content types:

- `text` / `text_delta`: only build the update when `chunk.text.length >
0`;
- string content: return `[]` when the string is empty.

The change is intentionally minimal and conservative — no information is
lost, and non-empty chunks are entirely unaffected.

## Tests

Three unit tests added on `toAcpNotifications`:

- empty string content → no notifications emitted;
- non-empty string content → still emitted unchanged;
- a stream containing an empty text block followed by a non-empty one →
only the non-empty block is emitted.

Full suite green (476 passing); eslint and prettier clean.

src/acp-agent.ts

@@ -4857,6 +4857,9 @@ export function toAcpNotifications(
   const registerHooks = options?.registerHooks !== false;
   const supportsTerminalOutput = options?.clientCapabilities?._meta?.["terminal_output"] === true;
   if (typeof content === "string") {
+    if (content.length === 0) {
+      return [];
+    }
     const update: SessionNotification["update"] = {
       sessionUpdate: role === "assistant" ? "agent_message_chunk" : "user_message_chunk",
       content: {
@@ -4886,13 +4889,15 @@ export function toAcpNotifications(
     switch (chunk.type) {
       case "text":
       case "text_delta":
-        update = {
-          sessionUpdate: role === "assistant" ? "agent_message_chunk" : "user_message_chunk",
-          content: {
-            type: "text",
-            text: chunk.text,
-          },
-        };
+        if (chunk.text.length > 0) {
+          update = {
+            sessionUpdate: role === "assistant" ? "agent_message_chunk" : "user_message_chunk",
+            content: {
+              type: "text",
+              text: chunk.text,
+            },
+          };
+        }
         break;
       case "image":
         update = {