Fix empty content deltas and leaked section markers in streaming

Tool parser:
- Case 3/4: return None instead of DeltaMessage(content='') when
  inside an open tool section with no parseable content yet.
  Empty-string content deltas pollute the response and break the
  content=null vs content='' contract with non-streaming.

Reasoning parser:
- Suppress tool-calls section markers from content forwarding.
  The tool parser detects them via current_text re-parsing; forwarding
  them as content causes double-handling.
- Already-past-reasoning path: strip section markers from content
  for the same reason.

kimi_k2_tool_parser.py

@@ -522,12 +522,17 @@ class KimiK2ToolParser(ToolParser):
             pre_section = current_text[len(previous_text):section_start_in_text]
             if pre_section.strip():
                 return DeltaMessage(content=pre_section)
-            return DeltaMessage(content="")
+            # No real content before the section — return None instead of
+            # an empty-string delta.  Empty content deltas confuse clients
+            # that distinguish content=null from content="".
+            return None
 
         # Case 4: Inside an open tool section but tool calls aren't
-        # parseable yet — emit empty delta to keep the stream alive.
+        # parseable yet — return None.  The serving layer will emit
+        # its own keep-alive if needed; we should not emit empty-string
+        # content deltas that pollute the response.
         if in_open_section:
-            return DeltaMessage(content="")
+            return None
 
         # Case 5: Section is closed and we're past it — forward any
         # new content that appeared after the section end marker.