cargo: verify --message-format=json passes through unfiltered

[PLNech]

Surfaced while triaging the agentic-fuzzer PR (#12) against v0.42.0.

cargo was rewritten in v0.42.0 around runner::RunOptions / streamed BlockStreamFilter. We should confirm that an explicit cargo build --message-format=json (or --message-format=json-diagnostic-rendered-ansi) passes through unfiltered, since downstream tooling parses that JSON stream and RTK's compaction would corrupt it.

Task: add a test (and passthrough guard if missing) for --message-format=json in src/cmds/rust/. Low effort, prevents a silent breakage for JSON consumers.