Fix stdin message splitting for back-to-back newline-delimited messages

[Copilot]

MockStdioResponsePlugin's @stdin.body.id placeholder fails to resolve when messages arrive in the same stdin.Read buffer — e.g., a JSON-RPC notification (no id) immediately followed by a request (with id). The entire multi-message buffer was passed as a single Body to the plugin pipeline, so JSON parsing would fail or only see the first message.

Changes

• Split buffered stdin by newlines before processing: ForwardStdinAsync now accumulates data in a StringBuilder, splits on \n, and dispatches each complete line individually through the plugin pipeline.
• Handle partial messages: Data not terminated by a newline is carried over to the next read.
• Safe multi-byte UTF-8 handling: Uses a stateful Decoder instead of raw Encoding.UTF8.GetString to avoid corrupting characters split across read boundaries.

Before/After

Before: a single stdin.Read returning {...notification...}\n{...request...}\n produced one StdioRequestArgs with the concatenated content as Body.

After: each newline-delimited message becomes its own StdioRequestArgs, so placeholder resolution sees the correct JSON for each message independently.