fix: exclude servers without a valid MCP tool listing

[gaojude]

Automatic discovery currently counts an ordinary HTTP catch-all server as a Next.js MCP server, even after its tool listing fails and returns zero tools. The response can report a successful discovery consisting entirely of unusable candidates.

Filter candidates after listing their MCP tools, and compute the count and no-server guidance from the remaining servers. This matches the existing explicit-port behavior. Adds a patch changeset.

Validation:

• The mixed-candidate and all-invalid regression cases failed against the original implementation.
• pnpm build, pnpm typecheck, and the complete pnpm exec vitest run: 36 tests passed.
• Started a real HTML catch-all on port 3001 alongside Next.js 16.3.4 on 3000 and 16.0.7 on 3456. Stdio discovery excluded 3001 and retained both Next.js servers; an explicit 3001 query also failed correctly.

Local validation used macOS arm64, Node.js 24.19.0, and pnpm 9.15.9. The uploaded GitHub-signed commit has the same Git tree as the tested checkout.