fix: tolerate missing text in streamed thinking chunks (#852)

## Summary

This makes streamed text/thinking chunk handling defensive when gateways
emit empty or malformed delta payloads.

- Guard `text_delta.text` and `thinking_delta.thinking` before reading
`.length`
- Skip missing/non-string text or thinking content the same way empty
chunks are already skipped
- Add regression coverage for a missing `thinking` delta before a real
text delta
- Add direct `toAcpNotifications` coverage for missing/non-string text
and thinking chunks

## Why

The adapter already skips empty thinking chunks, but some
Anthropic-compatible gateways can emit signature-only or omitted
thinking deltas where `thinking` is absent rather than an empty string.
In that case the adapter currently throws:

```text
Cannot read properties of undefined (reading 'length')
```

That aborts the ACP session even though the following text chunks are
valid.

This change keeps the existing behavior for valid chunks, while treating
missing/non-string chunk text as empty content.

## Test Plan

- `npm run check`
- `npm test -- --run`
- `npm run build`

Also verified manually through a VibeAround Web Chat flow using Claude
Code ACP with an Anthropic-compatible custom endpoint; the session
completed and returned a normal assistant response.

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

src/acp-agent.ts

@@ -2029,7 +2029,7 @@ export class ClaudeAcpAgent {
               // create a block the consolidated handler's `text.length > 0`
               // guard can never consume, stalling the diff cursor and
               // re-emitting the next block as a duplicate.
-              if (chunk && chunk.text.length > 0) {
+              if (chunk?.text) {
                 const index = message.event.index;
                 const last = streamedBlocks[streamedBlocks.length - 1];
                 if (last && last.index === index && last.type === chunk.type) {
@@ -5091,8 +5091,8 @@ export function toAcpNotifications(
     let update: SessionNotification["update"] | null = null;
     switch (chunk.type) {
       case "text":
-      case "text_delta":
-        if (chunk.text.length > 0) {
+      case "text_delta": {
+        if (chunk.text) {
           update = {
             sessionUpdate: role === "assistant" ? "agent_message_chunk" : "user_message_chunk",
             content: {
@@ -5102,6 +5102,7 @@ export function toAcpNotifications(
           };
         }
         break;
+      }
       case "image":
         update = {
           sessionUpdate: role === "assistant" ? "agent_message_chunk" : "user_message_chunk",
@@ -5114,10 +5115,10 @@ export function toAcpNotifications(
         };
         break;
       case "thinking":
-      case "thinking_delta":
+      case "thinking_delta": {
         // Recent models default `thinking.display` to "omitted", which streams
         // signature-only thinking blocks whose text is empty.
-        if (chunk.thinking.length > 0) {
+        if (chunk.thinking) {
           update = {
             sessionUpdate: "agent_thought_chunk",
             content: {
@@ -5127,6 +5128,7 @@ export function toAcpNotifications(
           };
         }
         break;
+      }
       case "tool_use":
       case "server_tool_use":
       case "mcp_tool_use": {