fix: preserve downstream MCP tool error status

[gaojude]

A downstream Next.js tool result with isError: true is currently wrapped in success: true, and the outer MCP result has no error flag. Clients can treat a failed runtime operation as successful.

Let nextjs_call return an MCP CallToolResult and preserve explicit downstream tool errors on the outer response. Request failures also set isError; healthy calls retain the existing JSON payload. The registration handler passes structured results through and continues wrapping string-returning tools. Adds a patch changeset.

Validation:

• Regression tests for downstream errors and request failures failed against the original implementation.
• pnpm build, pnpm typecheck, and the complete pnpm exec vitest run: 36 tests passed.
• Compared a real Next.js 16.3.4 compile_route failure directly with the stdio proxy: both report isError: true, and the proxy reports success: false.
• Verified healthy project metadata and connection-refused responses through the same MCP interface.

This preserves explicit MCP error semantics; it does not infer failure by parsing arbitrary text content returned by downstream tools.

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.