mcp: validate tool names (#640)

Implement SEP 986, enforcing restrictions on tool names. Since we're
already at v1 we couldn't turn this into a panic, so use an error log
instead. This is consistent with the typescript SDK. A note is left in
our 'rough edges' doc to panic in v2.

Fixes #621

mcp/server.go

@@ -191,6 +191,9 @@ func (s *Server) RemovePrompts(names ...string) {
 // Most users should use the top-level function [AddTool], which handles all these
 // responsibilities.
 func (s *Server) AddTool(t *Tool, h ToolHandler) {
+	if err := validateToolName(t.Name); err != nil {
+		s.opts.Logger.Error(fmt.Sprintf("AddTool: invalid tool name %q: %v", t.Name, err))
+	}
 	if t.InputSchema == nil {
 		// This prevents the tool author from forgetting to write a schema where
 		// one should be provided. If we papered over this by supplying the empty