mcp: fix setProgressToken when Meta is nil (#846)

When `SetProgressToken` is called on a `Params` struct with a nil `Meta`
field, the newly created map is never assigned back to the struct via
`SetMeta`, causing the progress token to be silently lost.

Call `p.SetMeta(m)` after creating the new map so the struct retains the
reference.

Fixes #845

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

mcp/shared.go

@@ -447,6 +447,7 @@ func setProgressToken(p Params, pt any) {
 	m := p.GetMeta()
 	if m == nil {
 		m = map[string]any{}
+		p.SetMeta(m)
 	}
 	m[progressTokenKey] = pt
 }