fix(weixin): keep multi-line messages in single bubble by default (#7903)

The Weixin adapter was splitting responses at every top-level newline,
causing notification spam (up to 70 API calls for a single long markdown
response). This salvages the best aspects of six contributor PRs:

Compact mode (new default):
- Messages under the 4000-char limit stay as a single bubble even with
  multiple lines, paragraphs, and code blocks
- Only oversized messages get split at logical markdown boundaries
- Inter-chunk delay (0.3s) between chunks prevents WeChat rate-limit drops

Legacy mode (opt-in):
- Set split_multiline_messages: true in platforms.weixin.extra config
- Or set WEIXIN_SPLIT_MULTILINE_MESSAGES=true env var
- Restores the old per-line splitting behavior

Salvaged from PRs #7797 (guantoubaozi), #7792 (luoxiao6645),
#7838 (qyx596), #7825 (weedge), #7784 (sherunlock03), #7773 (JnyRoad).
Core fix unanimous across all six; config toggle from #7838; inter-chunk
delay from #7825.

website/docs/user-guide/messaging/weixin.md

@@ -66,6 +66,9 @@ WEIXIN_ACCOUNT_ID=your-account-id
 WEIXIN_DM_POLICY=open
 WEIXIN_ALLOWED_USERS=user_id_1,user_id_2
 
+# Optional: restore legacy multiline splitting behavior
+# WEIXIN_SPLIT_MULTILINE_MESSAGES=true
+
 # Optional: home channel for cron/notifications
 WEIXIN_HOME_CHANNEL=chat_id
 WEIXIN_HOME_CHANNEL_NAME=Home
@@ -88,7 +91,7 @@ The adapter will restore saved credentials, connect to the iLink API, and begin
 - **AES-128-ECB encrypted CDN** — automatic encryption/decryption for all media transfers
 - **Context token persistence** — disk-backed reply continuity across restarts
 - **Markdown formatting** — headers, tables, and code blocks are reformatted for WeChat readability
-- **Smart message chunking** — long messages are split at logical boundaries (paragraphs, code fences)
+- **Smart message chunking** — messages stay as a single bubble when under the limit; only oversized payloads split at logical boundaries
 - **Typing indicators** — shows "typing…" status in the WeChat client while the agent processes
 - **SSRF protection** — outbound media URLs are validated before download
 - **Message deduplication** — 5-minute sliding window prevents double-processing
@@ -108,6 +111,7 @@ Set these in `config.yaml` under `platforms.weixin.extra`:
 | `group_policy` | `disabled` | Group access: `open`, `allowlist`, `disabled` |
 | `allow_from` | `[]` | User IDs allowed for DMs (when dm_policy=allowlist) |
 | `group_allow_from` | `[]` | Group IDs allowed (when group_policy=allowlist) |
+| `split_multiline_messages` | `false` | When `true`, split multi-line replies into multiple chat messages (legacy behavior). When `false`, keep multi-line replies as one message unless they exceed the length limit. |
 
 ## Access Policies
 
@@ -211,13 +215,14 @@ WeChat's personal chat does not natively render full Markdown. The adapter refor
 
 ## Message Chunking
 
-Long messages are split intelligently for chat delivery:
+Messages are delivered as a single chat message whenever they fit within the platform limit. Only oversized payloads are split for delivery:
 
 - Maximum message length: **4000 characters**
-- Split points prefer paragraph boundaries and blank lines
-- Code fences are kept intact (never split mid-block)
-- Indented continuation lines (sub-items in reformatted tables/lists) stay with their parent
+- Messages under the limit stay intact even when they contain multiple paragraphs or line breaks
+- Oversized messages split at logical boundaries (paragraphs, blank lines, code fences)
+- Code fences are kept intact whenever possible (never split mid-block unless the fence itself exceeds the limit)
 - Oversized individual blocks fall back to the base adapter's truncation logic
+- A 0.3 s inter-chunk delay prevents WeChat rate-limit drops when multiple chunks are sent
 
 ## Typing Indicators