[target-spec-miette] report serde_json errors properly

For errors parsing custom platforms, we should report the underlying
`serde_json` error as a proper diagnostic.

I had to deprecate the existing `Deserialize` variant because it didn't carry
the input (whoops!)

Cargo.toml

@@ -20,7 +20,9 @@ members = [
 
 [workspace.dependencies]
 ahash = "0.8.11"
+datatest-stable = "0.2.10"
 guppy-workspace-hack = "0.1.0"
+insta = "1.41.1"
 miette = "7.2.0"
 
 [workspace.package]

target-spec-miette/Cargo.toml

@@ -16,5 +16,15 @@ target-spec = { version = "3.2.2", path = "../target-spec" }
 miette.workspace = true
 guppy-workspace-hack.workspace = true
 
+[dev-dependencies]
+datatest-stable.workspace = true
+insta.workspace = true
+miette = { workspace = true, features = ["fancy"] }
+target-spec = { version = "3.2.2", path = "../target-spec", features = ["custom"] }
+
 [lints]
 workspace = true
+
+[[test]]
+name = "snapshot"
+harness = false