Response limiter drops structuredContent for large successful tool results

[tony]

Problem

Large successful tool results with output schemas can become protocol-invalid after response limiting.

FastMCP stores structured output in both content and structuredContent, then the response limiter measures the serialized whole result. libtmux-mcp uses a 50 KB backstop on large-output tools. When the limiter fires, the returned result can lose structuredContent, and FastMCP clients raise:

RuntimeError: Tool capture_pane has an output schema but did not return structured content

Minimal local reproduction

$ uv run python - <<'PY'
import asyncio
import uuid
from fastmcp import Client
from libtmux_mcp.server import build_mcp_server
from libtmux_mcp._utils import _get_server

async def main():
    socket_name = f"limit_{uuid.uuid4().hex[:8]}"
    server = _get_server(socket_name=socket_name)
    session = server.new_session(session_name="limit", kill_session=True, attach=False)
    pane = session.active_pane
    try:
        pane.send_keys("for i in $(seq 1 1400); do printf 'line%04d xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\\n' $i; done", enter=True)
        await asyncio.sleep(1.5)
        async with Client(build_mcp_server()) as client:
            await client.call_tool("capture_pane", {
                "pane_id": pane.pane_id,
                "socket_name": socket_name,
                "start": -1400,
                "max_lines": None,
            })
    finally:
        server.kill()

asyncio.run(main())
PY

Expected today: the client raises because the truncated response has no structured content.

Upstream mechanism

• FastMCP copies structured output into content when content is absent: https://github.com/jlowin/fastmcp/blob/v3.4.2/fastmcp_slim/fastmcp/tools/base.py#L96
• FastMCP emits structuredContent from ToolResult: https://github.com/jlowin/fastmcp/blob/v3.4.2/fastmcp_slim/fastmcp/tools/base.py#L143
• FastMCP response limiting measures serialized result size: https://github.com/jlowin/fastmcp/blob/v3.4.2/fastmcp_slim/fastmcp/server/middleware/response_limiting.py#L117-L119
• FastMCP's default limiter is 1 MB, much higher than libtmux-mcp's 50 KB cap: https://github.com/jlowin/fastmcp/blob/v3.4.2/fastmcp_slim/fastmcp/server/middleware/response_limiting.py#L53-L60

Minimal fix

Do not truncate schema-bearing success results into text-only results.

Preferred minimal path:

• Raise the middleware backstop above every tool-level cap.
• Keep truncation inside each tool's structured model fields (truncated, content, lines, etc.).
• Preserve structuredContent for all successful output-schema tools.

Acceptance criteria

• Large capture_pane, capture_since, snapshot_pane, and search_panes calls return valid structured content.
• FastMCP Client.call_tool() does not raise for large successful captures.
• Tests cover a large successful schema-bearing result through the client path.