hive-forge: dedup the --json refusal into refuse_json(), test it, cover attachment-get/artifact-get too

hive-forge/src/verbs/mod.rs

@@ -50,7 +50,7 @@ pub mod view;
 
 use std::fmt::Write as _;
 
-use anyhow::Result;
+use anyhow::{Result, bail};
 use forgejo_api::structs::{Attachment, Reaction};
 use serde::Deserialize;
 use serde_json::{Value, json};
@@ -67,6 +67,19 @@ pub(crate) fn print_json(v: &Value) -> Result<()> {
     Ok(())
 }
 
+/// Error out when the global `--json` flag is set on a verb that has no
+/// structured output to give it — a silently-ignored flag reads as "this is
+/// JSON" when it is actually prose or raw bytes, which is worse than a loud
+/// refusal. `why` names what the verb prints instead, e.g. `"only the plain
+/// name list"`; `verb` is the subcommand name as the operator typed it
+/// (`"branches"`, `"pr diff"`, …), for the error message.
+pub(crate) fn refuse_json(json_mode: bool, verb: &str, why: &str) -> Result<()> {
+    if json_mode {
+        bail!("hive-forge {verb}: --json is not supported for this verb — {why}");
+    }
+    Ok(())
+}
+
 /// Format an optional timestamp as its RFC 3339 string — the shape the
 /// raw API emitted, so output stays stable across the typed-client
 /// port. `None` (and the never-in-practice unformattable timestamp)
@@ -580,7 +593,20 @@ pub(crate) fn attachment_json(assets: Option<&[Attachment]>) -> Vec<Value> {
 
 #[cfg(test)]
 mod tests {
-    use super::{NullableVec, pct_encode, reviewed_older_head};
+    use super::{NullableVec, pct_encode, refuse_json, reviewed_older_head};
+
+    #[test]
+    fn refuse_json_bails_with_the_verb_and_reason() {
+        let err = refuse_json(true, "branches", "only the plain name list").unwrap_err();
+        let msg = err.to_string();
+        assert!(msg.contains("branches"), "{msg}");
+        assert!(msg.contains("only the plain name list"), "{msg}");
+    }
+
+    #[test]
+    fn refuse_json_passes_through_when_json_mode_is_off() {
+        assert!(refuse_json(false, "branches", "only the plain name list").is_ok());
+    }
 
     #[test]
     fn nullable_vec_treats_a_json_null_as_empty() {