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.

tests/gateway/test_weixin.py

@@ -62,15 +62,15 @@ class TestWeixinFormatting:
 
 
 class TestWeixinChunking:
-    def test_split_text_sends_top_level_newlines_as_separate_messages(self):
+    def test_split_text_keeps_short_multiline_message_in_single_chunk(self):
         adapter = _make_adapter()
 
         content = adapter.format_message("第一行\n第二行\n第三行")
         chunks = adapter._split_text(content)
 
-        assert chunks == ["第一行", "第二行", "第三行"]
+        assert chunks == ["第一行\n第二行\n第三行"]
 
-    def test_split_text_keeps_indented_followup_with_previous_line(self):
+    def test_split_text_keeps_short_reformatted_table_in_single_chunk(self):
         adapter = _make_adapter()
 
         content = adapter.format_message(
@@ -81,10 +81,7 @@ class TestWeixinChunking:
         )
         chunks = adapter._split_text(content)
 
-        assert chunks == [
-            "- Setting: Timeout\n  Value: 30s",
-            "- Setting: Retries\n  Value: 3",
-        ]
+        assert chunks == [content]
 
     def test_split_text_keeps_complete_code_block_together_when_possible(self):
         adapter = _make_adapter()
@@ -114,6 +111,23 @@ class TestWeixinChunking:
         assert all(len(chunk) <= adapter.MAX_MESSAGE_LENGTH for chunk in chunks)
         assert all(chunk.count("```") >= 2 for chunk in chunks)
 
+    def test_split_text_can_restore_legacy_multiline_splitting_via_config(self):
+        adapter = WeixinAdapter(
+            PlatformConfig(
+                enabled=True,
+                extra={
+                    "account_id": "acct",
+                    "token": "***",
+                    "split_multiline_messages": True,
+                },
+            )
+        )
+
+        content = adapter.format_message("第一行\n第二行\n第三行")
+        chunks = adapter._split_text(content)
+
+        assert chunks == ["第一行", "第二行", "第三行"]
+
 
 class TestWeixinConfig:
     def test_apply_env_overrides_configures_weixin(self):
@@ -127,6 +141,7 @@ class TestWeixinConfig:
                 "WEIXIN_BASE_URL": "https://ilink.example.com/",
                 "WEIXIN_CDN_BASE_URL": "https://cdn.example.com/c2c/",
                 "WEIXIN_DM_POLICY": "allowlist",
+                "WEIXIN_SPLIT_MULTILINE_MESSAGES": "true",
                 "WEIXIN_ALLOWED_USERS": "wxid_1,wxid_2",
                 "WEIXIN_HOME_CHANNEL": "wxid_1",
                 "WEIXIN_HOME_CHANNEL_NAME": "Primary DM",
@@ -142,6 +157,7 @@ class TestWeixinConfig:
         assert platform_config.extra["base_url"] == "https://ilink.example.com"
         assert platform_config.extra["cdn_base_url"] == "https://cdn.example.com/c2c"
         assert platform_config.extra["dm_policy"] == "allowlist"
+        assert platform_config.extra["split_multiline_messages"] == "true"
         assert platform_config.extra["allow_from"] == "wxid_1,wxid_2"
         assert platform_config.home_channel == HomeChannel(Platform.WEIXIN, "wxid_1", "Primary DM")