style(tool-service): fix pylint implicit-str-concat in _coerce_to_tool_result log

Black had collapsed the dict-envelope validation-failure warning into two
adjacent string literals on one line, which pylint flags as
``W1404 implicit-str-concat``. Merge the halves into a single literal.
No behavioural change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Jonathan Springer <jps@s390x.com>

Author: jonpspri

mcpgateway/services/tool_service.py

@@ -1667,7 +1667,7 @@ def _coerce_to_tool_result(self, payload: Any) -> ToolResult:
                 # upstream gets flagged in ops logs rather than silently
                 # downgraded to text.
                 logger.warning(
-                    "Dict payload matched MCP-envelope heuristics but failed ToolResult validation; " "falling back to text serialisation",
+                    "Dict payload matched MCP-envelope heuristics but failed ToolResult validation; falling back to text serialisation",
                     exc_info=True,
                 )