Fix Open WebUI MCP tool-call dispatch hang

Job description

Source issue: https://github.com/open webui/open webui/issues/26860 Bug summary: Open WebUI negotiates with a native MCP server successfully and receives a valid tool call from the model, but never dispatches the outbound tools/call request. The UI hangs on "Executing…" indefinitely. Direct curl calls to the same MCP server succeed, so this appears to be an internal Open WebUI tool dispatch or stream lifecycle bug. Scope: Reproduce the bug described in the GitHub issue. Identify the root cause between model tool call emission and MCP tools/call dispatch. Implement a fix so MCP tools/call requests are actually sent and completed. Verify behavior against the reported reproduction path. Acceptance criteria: The MCP tools/call request is dispatched correctly after tool selection. The chat no longer hangs indefinitely on "Executing…". Include a short implementation note with root cause, files changed, and validation steps. Prefer a PR ready patch or clean change set. Tech context: Likely fits TypeScript / React / MCP integration / debugging work.