docs(changelog): backfill 4 Python-specific divergences from non-parity table

Round-5 subagent found the one remaining gap: `docs/SELF_REVIEW.md` §5
requires a CHANGELOG entry for every divergence, and the non-parity
table has 8 Python-specific rows while the CHANGELOG had only 4
bullets. Backfill the missing four and broaden one existing bullet:

- Extended the `<url|text>` reserved-delimiters row to cover `]`,
  newlines, empty labels, schemeless URLs, and URLs containing
  `|`/`>` — all handled by the same emit-side fallback check, all
  accumulated over several commits during review iteration but only
  partially reflected in the CHANGELOG.
- Added dedicated bullets for Google Chat heading → bold rendering
  (upstream loses visual hierarchy by falling through to plain text).
- Added dedicated bullet for Google Chat image rendering (upstream
  silently drops the URL).
- Added bullet for fallback streaming's stream-exception capture +
  flush before re-raise (upstream propagates immediately, orphaning
  pendingEdit and stranding the placeholder).
- Added bullet for fallback streaming's `remend`'d final SentMessage
  content vs upstream's raw `accumulated`.

No code changes. Round-5 verdict on code: clean across all 7
principles. Only doc bookkeeping was missing.

https://claude.ai/code/session_01XE1bMoQ5BjCvgi1iLGKKhG

Author: claude

CHANGELOG.md

@@ -17,7 +17,11 @@ Synced to [Vercel Chat 4.26.0](https://github.com/vercel/chat).
 - **Fallback streaming clears stranded placeholders**: when a stream produces only whitespace with the default placeholder enabled, the final edit replaces `"..."` with `" "` so the message doesn't render as permanently loading. Upstream 4.26 intentionally leaves the placeholder visible to avoid empty-edit API calls; we issue one final edit to `" "` instead. Documented under [Known Non-Parity](docs/UPSTREAM_SYNC.md#known-non-parity-with-typescript-sdk).
 - **Google Chat `<url|text>` round-trip**: upstream 4.26 emits Google Chat's custom-label link syntax in the outgoing direction but doesn't parse it back in `to_ast()` / `extract_plain_text()`. A `[label](url)` posted through the gchat adapter would round-trip back as raw `"<url|label>"` text with no link node, breaking downstream handlers. We added the inverse regex to close the round-trip. Documented under Known Non-Parity.
 - **`from_json(data, adapter=X)` syncs `_adapter_name`**: upstream leaves `_adapterName` at the payload value even when an explicit adapter is bound, so `to_json()` can emit a stale name that refers to a different adapter than what runtime calls use. We update `_adapter_name = adapter.name` on explicit rebind so serialize and runtime stay consistent. Documented under Known Non-Parity.
-- **Google Chat link labels with `|` / `>` / newline**: Google Chat's `<url|text>` syntax has no escape for `|` or `>`. Upstream emits `<url|a > b>` verbatim, which truncates the label on the platform and can't round-trip back. We fall back to the pre-4.26 `text (url)` form whenever the label contains one of those reserved characters — the URL is still auto-linked by the platform and the label stays intact. Documented under Known Non-Parity.
+- **Google Chat `<url|text>` emit falls back to `text (url)` when it can't round-trip**: the custom-label syntax is only safe when the label doesn't contain `|` / `>` / `]` / newline, the label is non-empty, and the URL has an RFC 3986 scheme and no `|` or `>`. Upstream unconditionally emits `<url|text>`, producing malformed output for the edge cases. We fall back to `text (url)` (or bare URL for empty labels) so the content survives the round-trip and Google Chat's auto-link detection still fires for http(s) URLs. Documented under Known Non-Parity.
+- **Google Chat headings render as bold**: `#` / `##` / etc. emit as `*text*` for visual distinction. Upstream falls through to plain-text concatenation and loses the visual hierarchy entirely. Google Chat has no heading syntax, and bold is the closest approximation the platform supports. Documented under Known Non-Parity.
+- **Google Chat images render as `{alt} ({url})` (or bare URL)**: upstream has no image branch — the default fallback concatenates children only and silently drops the URL. We preserve the URL so the content isn't lost. Documented under Known Non-Parity.
+- **Fallback streaming captures stream exceptions and flushes before re-raising**: if the text stream iterator raises mid-flight (e.g. LLM connection drops), `_fallback_stream` now awaits `pending_edit`, flushes whatever partial content was rendered, clears the placeholder if appropriate, and THEN re-raises the original exception. Upstream propagates immediately, orphaning `pendingEdit` as a background task and stranding `"..."` on the message. Documented under Known Non-Parity.
+- **Fallback streaming final SentMessage carries repaired markdown**: the returned `SentMessage.markdown` is `renderer.finish()` output (`_remend`'d — inline markers auto-closed). Upstream ships raw `accumulated`. Narrow UX refinement — unobservable unless the stream ends mid-marker. Documented under Known Non-Parity.
 
 ## 0.4.25 (2026-04-10)