Test that `PlanningReport` can be serialized as JSON

[jgallagher]

While testing update on dublin, a Nexus instance went into maintenance after panicking multiple times with

thread 'tokio-runtime-worker' panicked at nexus/src/app/background/tasks/blueprint_planner.rs:295:31:
called `Result::unwrap()` on an `Err` value: Error("key must be a string", line: 0, column: 0)

and included this error log shortly before the panic:

{"msg":"could not serialize blueprint planning report","v":0,"name":"nexus","level":50,"time":"2025-09-24T18:19:01.819740006Z","hostname":"oxz_nexus_a4c68a8c-7e9a-45a4-8adc-88de59616d9d","pid":11682,"component":"datastore","component":"nexus","component":"ServerContext","name":"a4c68a8c-7e9a-45a4-8adc-88de59616d9d","file":"nexus/db-queries/src/db/datastore/deployment.rs:531","error":"key must be a string"}

#9080 fixes two maps that had non-string keys and removes the call to json!() that induced this panic, but does not add any test coverage for other maps I might've missed or that might be added in the future. @sunshowers suggested we add a proptest that generates arbitrary reports and confirms we can serialize them to JSON.