Fix CSR fallback crash when SSR returns non-string error

[m5r]

Problem

When a Node.js SSR worker crashes (e.g. V8 OOM with exit code 134), the nodejs library returns {:error, {:node_js_worker_exit, <tuple>}} instead of {:error, "string message"}.

In send_response/1, this matches {:error, message} where message is a tuple. The Logger call on line 845 does "...\n\n#{message}", which calls String.Chars.to_string/1 on the tuple and raises Protocol.UndefinedError. The CSR fallback on the next line never executes, and the user gets a 500.

The raise_on_ssr_failure path has the same issue: RenderError expects message to be a string.

Fix

Coerce message with inspect/1 when it's not a binary, before logging or raising:

{:error, message} ->
  message = if is_binary(message), do: message, else: inspect(message)

Test

Added test/js/ssr-crash.js that calls process.exit(134) to simulate a worker crash. The test asserts the request still returns 200 (CSR fallback) instead of crashing.