fix(stdio): iterate over decoded messages list

[b-serra]

Summary

Fixes #240

Message.decode/1 returns a list of messages, but process_message/2 expects a single message (map). This caused a BadMapError when processing any incoming stdio message.

Changes

• Iterate over the decoded messages list using Enum.each/2 before calling process_message/2 for each message

Before (broken)

case Message.decode(data) do
  {:ok, messages} ->
    process_message(messages, state)  # messages is a list, crashes

After (fixed)

case Message.decode(data) do
  {:ok, messages} ->
    Enum.each(messages, &process_message(&1, state))  # properly handles list

Testing

• All existing tests pass (442 tests, 1 unrelated failure in HTTP transport)
• Manually tested with MCP stdio server - initialize requests now work correctly