`SetProgressToken` silently fails when `Meta` is nil

[StevenRChen]

Describe the bug
SetProgressToken in mcp/shared.go silently fails when called on a Params struct with a nil Meta field. The progress token is written into a locally created map that is never assigned back to the struct, so it is immediately discarded.

To Reproduce
Minimum test case

p := &mcp.CallToolParams{Name: "n"}
p.SetProgressToken("t")
fmt.Println(p.GetProgressToken()) // nil

Expected behaviour
GetProgressToken() should return "t" after calling SetProgressToken("t"), regardless of whether Meta was previously set.

[StevenRChen]

#838 is fixing the same issue.