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/protocol_test.go

@@ -64,6 +64,16 @@ func TestParamsMeta(t *testing.T) {
 		t.Errorf("got %v, want `t`", g)
 	}
 
+	// The GetProgressToken and SetProgressToken methods work on a params struct that doesn't have a Meta field.
+	if g := p2.GetProgressToken(); g != nil {
+		t.Errorf("got %v, want nil", g)
+	}
+
+	p2.SetProgressToken("t")
+	if g := p2.GetProgressToken(); g != "t" {
+		t.Errorf("got %v, want `t`", g)
+	}
+
 	// You can set a progress token to an int, int32 or int64.
 	p.SetProgressToken(int(1))
 	p.SetProgressToken(int32(1))