Code mode: three self-hosted usability bugs (i18n hint, Test button, codeMode write path) #1444

Found while enabling code mode on a self-hosted instance. All three affect self-hosted usability; none is a schema change.

1. Tool editor shows the raw key toolEditor.codeUsesHint.
The message in packages/frontend/messages/{en,de,it}.json contained a literal JSON example with { } braces (and <id> angle brackets). next-intl/ICU reads { as a placeholder and <id> as a tag, so formatting throws and falls back to the key. Fix: reword the hint without braces/angle-brackets.

2. The dashboard "Test" button on a saved code tool does not run the program.
POST /api/connectors/:id/tools/:toolId/test called connectorsService.executeConnectorCall, the plain static/REST engine, which has no method: 'code' branch — so a code tool tried to HTTP-call the connector's base URL and failed, never reaching the runner. Fix: route method === 'code' to the out-of-process code path with the same governance a real call uses.

3. The per-server codeMode setting cannot be set through the API, only in the DB.
codeMode was absent from CreateMcpServerDto/UpdateMcpServerDto and had no frontend control, so a self-hosted admin could not turn code mode on without raw SQL. Fix: add codeMode to the update DTO (@IsIn(['off','tools','inline'])), persist it, and add a select on the MCP server settings page.

Behaviour unchanged on Cloud and self-hosted until an operator runs the code-runner and a workspace turns codeMode on; off by default everywhere.