fix(web): break /mcp trailing-slash redirect loop that made the MCP endpoint unreachable

[arozumenko]

Fixes #352.

Problem

https://wikis.onetest.ai/mcp was unreachable from MCP clients — ConnectionRefused. It's a trailing-slash redirect loop, not network/auth:

• The Starlette Mount("/mcp", …) (streamable route at /) 307-redirects bare /mcp → /mcp/. Because web/src/middleware.ts rewrote to the raw upstream, the backend built that redirect from its own request URL and leaked the internal host: location: http://backend:8000/mcp/ — unreachable by the client.
• Next.js (trailingSlash: false) 308-redirects /mcp/ → /mcp, so the other direction loops.

Fix

Proxy the MCP path to the trailing-slash form the Mount expects, so the backend never issues the redirect. NextResponse.rewrite keeps the client URL as /mcp (no client-visible redirect, no loop).

if (pathname === '/mcp' || pathname.startsWith('/mcp/')) {
  const path = pathname === '/mcp' ? '/mcp/' : pathname;
  const url = new URL(path + request.nextUrl.search, BACKEND_URL);
  return NextResponse.rewrite(url);
}

Client config stays https://wikis.onetest.ai/mcp (no trailing slash) — the bug was purely server-side.

Verification

• Before: curl -i https://wikis.onetest.ai/mcp → 307 location: http://backend:8000/mcp/; curl -i .../mcp/ → 308 location: /mcp.
• After deploy, the initialize POST to /mcp should return 200 with an SSE/JSON response and no Location header. (Requires a deploy to verify end-to-end.)

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