Tool parser: fallback to <|tool_call_begin|> when no section marker

Some Kimi K2.5 model variants (nvidia/Kimi-K2.5-NVFP4) omit
<|tool_calls_section_begin|> and go directly to <|tool_call_begin|>.
The tool parser was only looking for section-level markers, so these
tool calls were forwarded as raw content text instead of being parsed.

Fix: _find_section_start and _find_section_start_end now fall back to
<|tool_call_begin|> as a section start when no section-level marker
is found. The section end falls back to <|tool_call_end|>.

kimi_k2_tool_parser.py

@@ -109,6 +109,9 @@ class KimiK2ToolParser(ToolParser):
             "<|tool_calls_section_end|>",
             "<|tool_call_section_end|>",
         ]
+        # Some model variants omit the section-level marker and go
+        # directly to <|tool_call_begin|>.  Treat it as a fallback.
+        self._fallback_section_start: str = "<|tool_call_begin|>"
         # Primary variant for ToolParser base class / adjust_request
         self.tool_calls_start_token: str = "<|tool_calls_section_begin|>"
         self.tool_calls_end_token: str = "<|tool_calls_section_end|>"
@@ -209,12 +212,23 @@ class KimiK2ToolParser(ToolParser):
         return function_name, raw_id
 
     def _find_section_start(self, text: str) -> int:
-        """Return the index of the first section-start marker, or -1."""
+        """Return the index of the first section-start marker, or -1.
+
+        Falls back to <|tool_call_begin|> if no section-level marker
+        is found.  Some model variants skip <|tool_calls_section_begin|>
+        and go directly to <|tool_call_begin|>.
+        """
         best = -1
         for variant in self.tool_calls_section_start_variants:
             idx = text.find(variant)
             if idx != -1 and (best == -1 or idx < best):
                 best = idx
+        # Fallback: if no section-level marker found, look for
+        # <|tool_call_begin|> directly.
+        if best == -1 and self._fallback_section_start:
+            idx = text.find(self._fallback_section_start)
+            if idx != -1:
+                best = idx
         return best
 
     def _find_section_start_end(self, text: str) -> tuple[int, int]:
@@ -223,6 +237,9 @@ class KimiK2ToolParser(ToolParser):
         *start_of_inner* points just past the section-start marker.
         *end_of_inner* is the index of the section-end marker, or -1
         if the section is still open.
+
+        Falls back to <|tool_call_begin|> if no section-level marker
+        is found.
         """
         for variant in self.tool_calls_section_start_variants:
             idx = text.find(variant)
@@ -234,6 +251,20 @@ class KimiK2ToolParser(ToolParser):
                     if end_idx != -1:
                         return inner_start, end_idx
                 return inner_start, -1
+
+        # Fallback: no section-level marker found.  Look for
+        # <|tool_call_begin|> directly as the section start.
+        if self._fallback_section_start:
+            idx = text.find(self._fallback_section_start)
+            if idx != -1:
+                inner_start = idx + len(self._fallback_section_start)
+                # Look for <|tool_call_end|> as the section end
+                end_marker = self._fallback_section_start.replace("begin", "end")
+                end_idx = text.find(end_marker, inner_start)
+                if end_idx != -1:
+                    return inner_start, end_idx
+                return inner_start, -1
+
         return -1, -1
 
     # ------------------------------------------------------------------