Route MCP requests through an explicit rejection handler

The Obsidian plugin review rejects undescribed eslint-disable
directives, and the no-misused-promises suppression on the MCP route
was the last one remaining. Replace the async route handler with a
plain handler that forwards rejections to Express via .catch(next),
matching the pattern already used by the handle() helper for every
other route. This also improves behavior: rejections now reach the
error-handling middleware instead of being swallowed by Express 4's
promise-unaware router.

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