fix(chat): stop code-block button flicker during streaming (#3023)

Render streamed markdown incrementally (freeze finalized blocks,
re-render only the growing tail) instead of re-rendering the whole
message every token, which recreated every <pre> and dropped CSS :hover.

tests/test_streaming_segmenter_js.py

@@ -0,0 +1,38 @@
+"""Runs the Node-based streaming-render segmenter suite (tests/streaming/*.test.mjs).
+
+Covers the pure incremental-render segmenter (static/js/streamingSegmenter.js):
+unit boundaries plus a streaming-invariant fuzz that feeds a markdown corpus in
+token-by-token and asserts the freeze/tail split always matches a single full
+render. Pure JS — no DOM, no extra dependencies. Skipped when node is
+unavailable, mirroring tests/test_markdown_rendering_js.py.
+
+The renderer's DOM behavior (streamingRenderer.js) is exercised against a running
+app, not here, consistent with how this project tests browser-coupled code.
+"""
+
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_HAS_NODE = shutil.which("node") is not None
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_streaming_segmenter_suite():
+    test_files = sorted(str(p) for p in (_REPO / "tests" / "streaming").glob("*.test.mjs"))
+    assert test_files, "no streaming test files found"
+
+    result = subprocess.run(
+        ["node", "--test", *test_files],
+        cwd=_REPO,
+        capture_output=True,
+        timeout=180,
+        text=True,
+    )
+    if result.returncode != 0:
+        raise AssertionError(
+            f"node --test failed:\nSTDOUT:\n{result.stdout}\nSTDERR:\n{result.stderr}"
+        )