feat(arr): OpenAI-compatible translation backend

Behind the existing translate-openai feature. One JSON-in/JSON-out
chat/completions request per batch; refusal, content-filter and
truncated replies fail loudly instead of validating as a short batch.

crates/arr-subs/Cargo.toml

@@ -34,5 +34,12 @@ tempfile.workspace = true
 tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
 wiremock.workspace = true
 
+# Only built with the feature it exercises — cargo skips the target
+# otherwise, rather than compiling an empty one that flags every crate
+# dependency as unused.
+[[test]]
+name = "openai"
+required-features = ["translate-openai"]
+
 [lints]
 workspace = true