fix: Prevent MCPToolset GC during toolset add

[vblagoje]

Why:

Fixes a garbage collection issue that prevented users from properly combining multiple MCPToolset instances using the inline syntax tools.add(MCPToolset(...)). When MCPToolset objects were created inline and added to another toolset, Python's garbage collector would prematurely destroy the temporary toolset objects, closing their MCP connections and leaving orphaned tools that failed with "Not connected to an MCP server" errors.

• fixes Add multi server support to MCPTool and MCPToolset. #1986

What:

• Useowner_toolset parameter to maintain strong references to prevent garbage collection

How can it be used:

# Previously broken - temporary MCPToolset would be garbage collected
server_info = SSEServerInfo(url="https://mcp.amap.com/sse?key=xxx")
htw_server_info = SSEServerInfo(url="http://127.0.0.1:8333/sse")
tools = MCPToolset(server_info=htw_server_info)
tools.add(MCPToolset(server_info=server_info))  # Now works correctly!

# Alternative approach that always worked
tools_a = MCPToolset(server_info=htw_server_info)
tools_b = MCPToolset(server_info=server_info)
all_tools = tools_a + tools_b  # Still works

How did you test it:

• Manual confirmation of the original issue example
• Using itinerary agent with the previously "broken" setup of MCPToolset usage
• Ran full MCP integration test suite to ensure no regressions in existing functionality

Notes for the reviewer:

N/A

[vblagoje]

Hey @Amnah199 do you have time to take a look at this one today?

[Amnah199]

@vblagoje taking a look now 👍

[Amnah199]

LG already! Just a small comment.

[vblagoje]

@Amnah199 can you please take another look?

[Amnah199]

Lets go!