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_reasoning_parser.py

@@ -249,6 +249,13 @@ class KimiK2ReasoningParser(ReasoningParser):
         if self.is_reasoning_end(previous_token_ids):
             # Strip any residual think tags that might appear in content
             cleaned = self._strip_think_tags(delta_text)
+            if not cleaned:
+                return None
+            # If tool-calls section markers are present, suppress them
+            # from content — the tool parser handles them via current_text
+            # re-parsing and does not need them forwarded as content.
+            for variant in self._tool_section_start_variants:
+                cleaned = cleaned.replace(variant, "")
             return DeltaMessage(content=cleaned) if cleaned else None
 
         # ── Check for </think> in this delta ──
@@ -270,15 +277,13 @@ class KimiK2ReasoningParser(ReasoningParser):
         tool_idx = self._find_tool_section_start(delta_text)
         if tool_idx != -1:
             reasoning = self._strip_think_tags(delta_text[:tool_idx])
-            # Forward the tool section marker as content so the tool
-            # parser can detect it.
-            content = delta_text[tool_idx:]
-
+            # Do NOT forward the tool section marker as content.  The
+            # tool parser detects it via current_text re-parsing on its
+            # own.  Forwarding it causes double-handling and empty content
+            # deltas.
             kwargs = {}
             if reasoning:
                 kwargs["reasoning"] = reasoning
-            if content:
-                kwargs["content"] = content
             return DeltaMessage(**kwargs) if kwargs else None
 
         # ── Still in reasoning — strip <think> tag if present ──