fix: stop running marked.parse on every streaming text delta

The streaming path used to call _renderAssistantMarkdown on every
incoming text delta, which re-ran marked.parse on the WHOLE
accumulated markdown source and re-post-processed (added the
Copy/Insert/Replace toolbar to) every <pre> block.

This is a partial-render problem: a markdown source mid-stream (e.g.
an opening ```py fence with no closing fence yet, or a partial
heading) classifies DIFFERENTLY than the complete message. Each
new delta could re-classify the same text into a different DOM
shape, causing visible flicker - <pre>s appearing and disappearing,
the toolbar popping in and out, paragraphs re-grouping - while the
model is still typing. The bug surfaced because the user could see
the glitchy view during a stream but everything looked correct
after closing+reopening the panel (which goes through the static
setMessages -> marked.parse path on the full text).

Fix: keep the streaming path dumb. _appendToAssistantText now just
appends the raw delta to the assistant element's textContent. The
full markdown render happens exactly once at the end of the turn,
inside _resetStreamPointers (which the prompt invokes when
session.idle arrives, before calling onStreamEnd). Result:

- The user still sees real-time text growth (textContent += delta).
- No more intermediate-state flicker.
- The rendered view at turn end is byte-for-byte equivalent to what
  setMessages would produce from the stored history (they share
  _renderAssistantMarkdown).

The corresponding regression test is updated: it now asserts the
DURING-stream state is raw text (no <pre>, no toolbar) and the
POST-idle state is the rendered DOM with the toolbar attached.

Tests: 72 jest (unchanged count, the one streaming test was
rewritten), 52 pytest, build green.

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

src/components/opencode_inline_prompt.ts

@@ -596,16 +596,19 @@ export class OpenCodeInlinePrompt extends Widget {
     if (!this._currentAssistantEl) {
       this._currentAssistantEl = this._createMessageElement('assistant', '');
     }
-    // Accumulate the raw markdown source, then re-render the whole
-    // message through marked.parse. This way the user sees properly
-    // formatted output (code blocks as <pre><code>, headings, lists)
-    // in real-time as text deltas arrive — NOT the raw ```fence```
-    // source that an append-only .textContent would show.
+    // During streaming, append the raw delta to the assistant element's
+    // textContent. We do NOT run marked.parse on every delta: the
+    // partial source (e.g. an opening ```py fence with no closing
+    // fence yet) is structurally different from the complete message
+    // and the toolbar post-processing creates visible flicker —
+    // <pre>s appear and disappear as the renderer re-classifies the
+    // incomplete markdown between deltas. The user gets stable
+    // real-time text here, and the full markdown render happens once
+    // at the end of the turn (see _resetStreamPointers) so the result
+    // is identical to what setMessages would produce from the stored
+    // history.
     this._currentAssistantText += text;
-    this._renderAssistantMarkdown(
-      this._currentAssistantEl,
-      this._currentAssistantText
-    );
+    this._currentAssistantEl.textContent = this._currentAssistantText;
     this._scrollToBottom();
   }
 
@@ -807,6 +810,19 @@ export class OpenCodeInlinePrompt extends Widget {
   }
 
   private _resetStreamPointers(): void {
+    // Finalize the in-progress assistant message by running the
+    // markdown render ONCE on the accumulated raw text. This is the
+    // moment where the streaming path converges to the same DOM
+    // structure that setMessages would produce from the stored
+    // history (i.e. identical to "reopen" behavior). Without this
+    // final pass the user would see raw ```fence``` source
+    // permanently until they close+reopen the panel.
+    if (this._currentAssistantEl && this._currentAssistantText) {
+      this._renderAssistantMarkdown(
+        this._currentAssistantEl,
+        this._currentAssistantText
+      );
+    }
     this._currentAssistantEl = null;
     this._currentAssistantText = '';
     this._activeBlocks = {};