Cover the MCP transport's oversized-line bound

[kikashy]

Goal

Test what the stdio MCP server does with a JSON-RPC line larger than maxMessageBytes.

Why

internal/mcp/server.go bounds one line at 16 MiB and sizes the scanner buffer accordingly:

const maxMessageBytes = 16 * 1024 * 1024
scanner.Buffer(make([]byte, 0, 64*1024), maxMessageBytes)

maxMessageBytes appears nowhere in internal/mcp/server_test.go. A client that sends an oversized line gets whatever bufio.Scanner does at its limit, and nothing pins whether that is a clean refusal or a silently truncated read that the server then tries to parse.

That distinction matters here more than usual: a truncated line that happens to parse would be a different request than the one sent.

Scope

• Change internal/mcp/server_test.go only.
• Make the bound overridable in the test the way maxMatrixResultBytes already is (a var rather than a const) if that is the smallest change; otherwise construct an oversized line directly.
• Cover:
  • a line over the bound;
  • whether the server emits a response, and if so which;
  • whether the stream continues or ends after it.
• Assert what the server does today rather than changing it. If the current behaviour is judged wrong, say so in the pull request and stop there.

Acceptance criteria

• An oversized line has a test that states the observed behaviour explicitly.
• The test says whether a response is emitted and whether processing continues.
• No production change unless the pull request argues for one separately.
• env GO111MODULE=on go test ./internal/mcp and go test ./... pass.
• Every commit includes a DCO sign-off created with git commit -s.

Contributor learning

Line-oriented transports, bufio.Scanner limits, and the difference between refusing an input and processing a prefix of it.