Official MCP client: connection and discovery timeouts are not part of timeoutMs

Follow-up from the PR #1031 review (Codex finding 5).

`callOfficialTool` starts the `timeoutMs` deadline at the `callTool` request, but a cold client first connects and lists tools, and each of those steps has its own 30 s timeout inside `N8nOfficialMcpClient`. A first call on a cold connection can therefore overrun its declared `timeoutMs` by up to about 60 s.

Suggested fix: propagate one absolute deadline through connect, `listTools` and `callTool` (subtract elapsed time at each step), and document that `timeoutMs` is the whole-call budget.

Conceived by Romuald Członkowski - www.aiadvisors.pl/en