fix(server): survive malformed JSON-RPC frames instead of exiting

[charliewwdev]

Fixes #54.

Summary

A single malformed frame from any client ended the MCP session. Reproduced against main with a stdio probe; two independent defects were responsible.

1. Invalid UTF-8 killed the process. stdin.transform(utf8.decoder) raises FormatException on the stream, not inside the per-line callback, and listen() had no onError. The exception was unhandled and terminated the isolate:

[ERR] Unhandled exception:
[ERR] FormatException: Invalid UTF-8 byte (at offset 0)
### exited=255

That is the unresponsive after malformed input (eof) verdict in the report.

2. Parse errors were silently dropped. Truncated JSON was caught, but the reply went through _sendError(null, -32700, ...), and _sendError opens with if (id == null) return;. The guard is correct for notifications and wrong for parse errors — JSON-RPC 2.0 requires them to be answered with a null id. The client saw silence and concluded the server was hung.

Changes

• Decode with Utf8Decoder(allowMalformed: true) so bad bytes become U+FFFD and degrade into an ordinary parse error instead of a stream failure.
• Add onError and cancelOnError: false to the stdin subscription so no stream-level error can end the session.
• Add _sendProtocolError(code, message) for the null-id replies the spec requires, leaving _sendError's notification behaviour untouched.
• Answer -32600 Invalid Request for frames that are valid JSON but not request objects (previously ignored with no reply).
• run() now awaits stdin closing. It used to return as soon as the listener was attached, so runServer released the single-instance lock while the server was still live.

Test plan

• New test/server_malformed_frame_test.dart drives a real server subprocess through truncated JSON, non-JSON text, invalid UTF-8 bytes, and a JSON array, asserting each is answered and that a following tools/list still succeeds. A fifth test pins that notifications stay unanswered.
• Verified the tests are not vacuous: 4 of 5 fail on main, 5/5 pass with the fix. (The notification test passes on both, as intended.)
• flutter analyze lib/src/cli/server.dart test/server_malformed_frame_test.dart — no issues.
• Manual probe: 6 hostile frames in a row, server answers each and continues serving.

--- MALFORMED json (truncated)
[OUT] {"jsonrpc":"2.0","id":null,"error":{"code":-32700,...}}
--- INVALID UTF-8 bytes
[OUT] {"jsonrpc":"2.0","id":null,"error":{"code":-32700,...}}
--- JSON array (not an object)
[OUT] {"jsonrpc":"2.0","id":null,"error":{"code":-32600,...}}
--- notification (no id, must be silent)
--- valid tools/list
[OUT] {"jsonrpc":"2.0","id":9,"result":{"tools":[...]}}
### exited=None  alive=True

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