Add 87 unit tests for uncovered pure logic in arc-workflows

Cover format_cost, format_tokens_human, compute_stage_cost, all
format_event_summary variants, validate_command, context_diff, and
parse_label_filters with fast unit tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/cli/runs.rs

@@ -620,4 +620,41 @@ mod tests {
         prune_from(&args, base).unwrap();
         assert!(!orphan_dir.exists(), "orphan directory should be deleted");
     }
+
+    #[test]
+    fn parse_label_filters_basic() {
+        let args = vec!["env=prod".to_string()];
+        let result = parse_label_filters(&args);
+        assert_eq!(result, vec![("env".to_string(), "prod".to_string())]);
+    }
+
+    #[test]
+    fn parse_label_filters_multiple() {
+        let args = vec!["a=1".to_string(), "b=2".to_string()];
+        let result = parse_label_filters(&args);
+        assert_eq!(result.len(), 2);
+        assert!(result.contains(&("a".to_string(), "1".to_string())));
+        assert!(result.contains(&("b".to_string(), "2".to_string())));
+    }
+
+    #[test]
+    fn parse_label_filters_value_with_equals() {
+        let args = vec!["key=a=b".to_string()];
+        let result = parse_label_filters(&args);
+        assert_eq!(result, vec![("key".to_string(), "a=b".to_string())]);
+    }
+
+    #[test]
+    fn parse_label_filters_skips_no_equals() {
+        let args = vec!["nope".to_string(), "a=1".to_string()];
+        let result = parse_label_filters(&args);
+        assert_eq!(result, vec![("a".to_string(), "1".to_string())]);
+    }
+
+    #[test]
+    fn parse_label_filters_empty() {
+        let args: Vec<String> = vec![];
+        let result = parse_label_filters(&args);
+        assert!(result.is_empty());
+    }
 }

crates/arc-workflows/src/cli/validate.rs

@@ -31,3 +31,61 @@ pub fn validate_command(args: &ValidateArgs, styles: &Styles) -> anyhow::Result<
     eprintln!("Validation: {}", styles.green.apply_to("OK"));
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Write;
+    use std::path::PathBuf;
+
+    #[test]
+    fn validate_valid_workflow() {
+        let mut tmp = tempfile::NamedTempFile::new().unwrap();
+        write!(
+            tmp,
+            r#"digraph Simple {{
+    graph [goal="Run tests and report results"]
+    rankdir=LR
+
+    start [shape=Mdiamond, label="Start"]
+    exit  [shape=Msquare, label="Exit"]
+
+    run_tests [label="Run Tests", prompt="Run the test suite and report results"]
+    report    [label="Report", prompt="Summarize the test results"]
+
+    start -> run_tests -> report -> exit
+}}"#
+        )
+        .unwrap();
+
+        let args = ValidateArgs {
+            workflow: tmp.path().to_path_buf(),
+        };
+        let styles = Styles::new(false);
+        let result = validate_command(&args, &styles);
+        assert!(result.is_ok(), "expected Ok but got: {result:?}");
+    }
+
+    #[test]
+    fn validate_invalid_syntax() {
+        let mut tmp = tempfile::NamedTempFile::new().unwrap();
+        write!(tmp, "not a valid dot file").unwrap();
+
+        let args = ValidateArgs {
+            workflow: tmp.path().to_path_buf(),
+        };
+        let styles = Styles::new(false);
+        let result = validate_command(&args, &styles);
+        assert!(result.is_err(), "expected Err for invalid syntax");
+    }
+
+    #[test]
+    fn validate_missing_file() {
+        let args = ValidateArgs {
+            workflow: PathBuf::from("/tmp/nonexistent_workflow_12345.dot"),
+        };
+        let styles = Styles::new(false);
+        let result = validate_command(&args, &styles);
+        assert!(result.is_err(), "expected Err for missing file");
+    }
+}

crates/arc-workflows/src/handler/manager_loop.rs

@@ -595,4 +595,44 @@ mod tests {
     fn parse_duration_str_invalid_fallback() {
         assert_eq!(parse_duration_str("bad"), Duration::from_secs(45));
     }
+
+    #[test]
+    fn context_diff_detects_additions() {
+        let before = HashMap::new();
+        let mut after = HashMap::new();
+        after.insert("key".to_string(), serde_json::json!("value"));
+        let diff = context_diff(&before, &after);
+        assert_eq!(diff.len(), 1);
+        assert_eq!(diff.get("key"), Some(&serde_json::json!("value")));
+    }
+
+    #[test]
+    fn context_diff_detects_changes() {
+        let mut before = HashMap::new();
+        before.insert("key".to_string(), serde_json::json!("old"));
+        let mut after = HashMap::new();
+        after.insert("key".to_string(), serde_json::json!("new"));
+        let diff = context_diff(&before, &after);
+        assert_eq!(diff.len(), 1);
+        assert_eq!(diff.get("key"), Some(&serde_json::json!("new")));
+    }
+
+    #[test]
+    fn context_diff_ignores_unchanged() {
+        let mut before = HashMap::new();
+        before.insert("key".to_string(), serde_json::json!("same"));
+        let mut after = HashMap::new();
+        after.insert("key".to_string(), serde_json::json!("same"));
+        let diff = context_diff(&before, &after);
+        assert!(diff.is_empty());
+    }
+
+    #[test]
+    fn context_diff_ignores_deletions() {
+        let mut before = HashMap::new();
+        before.insert("removed".to_string(), serde_json::json!("gone"));
+        let after = HashMap::new();
+        let diff = context_diff(&before, &after);
+        assert!(diff.is_empty());
+    }
 }