fix: stop leaking exception messages to clients via JSON-RPC error data

Tool handler exceptions and anything reaching JsonRpcHandler's blind
rescue had their #message echoed into the error data member, exposing
internals to untrusted clients (CWE-209). Capability errors keep their
client-facing message by re-raising through RequestHandlerError, the
one channel whose message is deliberately surfaced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/json_rpc_handler_test.rb

@@ -310,7 +310,6 @@ describe JsonRpcHandler do
       assert_rpc_error expected_error: {
         code: -32603,
         message: "Internal error",
-        data: "Something bad happened",
       }
       assert_equal 1, @response[:id]
       assert_nil @response[:result]
@@ -452,7 +451,6 @@ describe JsonRpcHandler do
       assert_rpc_error expected_error: {
         code: -32603,
         message: "Internal error",
-        data: "Something bad happened",
       }
     end