fix(streaming): strip Gemma 4 thinking token delimiter in all paths — closes #607

Fixes <|turn|>thinking delimiter (was wrong as <|turn>thinking) in api/streaming.py, static/messages.js, and static/ui.js. Adds 13 regression tests. Independent review by @nesquena.

tests/test_sprint38.py

@@ -57,6 +57,35 @@ def test_gemma_content_removal_uses_replace_not_slice():
         "ui.js must call .trimStart() on content after removing the Gemma channel block"
 
 
+def test_gemma_turn_regex_in_ui_js():
+    """The Gemma 4 <|turn|>thinking\\n...<turn|> pattern must be extracted from persisted content."""
+    # Detection in _messageHasReasoningPayload (correct double-pipe format)
+    assert "<\\|turn\\|>thinking" in UI_JS, (
+        "ui.js _messageHasReasoningPayload must detect Gemma 4 <|turn|>thinking\\n...<turn|> pattern"
+        " (note: double-pipe: <|turn|> not <|turn>)"
+    )
+    # Extraction block
+    match = re.search(r'const gemmaTurnMatch=content\.match\((/[^/]+/)\)', UI_JS)
+    assert match, "gemmaTurnMatch line not found in ui.js"
+    pattern = match.group(1)
+    assert not pattern.startswith('/^'), (
+        f"gemmaTurnMatch regex must not use ^ anchor — got {pattern}"
+    )
+
+
+def test_gemma_turn_content_removal_uses_replace_not_slice():
+    """Gemma 4 turn token removal must use .replace() not .slice()."""
+    idx = UI_JS.find("if(gemmaTurnMatch){")
+    assert idx >= 0, "gemmaTurnMatch handler block not found in ui.js"
+    block = UI_JS[idx:idx+240]
+    assert "content.replace(" in block, (
+        "ui.js must use content.replace() to remove Gemma 4 turn block (not .slice())"
+    )
+    assert ".trimStart()" in block, (
+        "ui.js must call .trimStart() on content after removing the Gemma 4 turn block"
+    )
+
+
 # ── messages.js: streaming render path ───────────────────────────────────────
 
 def test_stream_display_trims_before_startswith():