enhancement: check VRL conditions return type at compile time (vector…

…dotdev#17894)

closes: vectordotdev#17175 

As discussed offline with @fuchsnj, we do not need to handle fallible
conditions because providing a fallible condition already results in an
error.

src/conditions/vrl.rs

@@ -63,6 +63,10 @@ impl ConditionalConfig for VrlConfig {
                 .to_string()
         })?;
 
+        if !program.final_type_info().result.is_boolean() {
+            return Err("VRL conditions must return a boolean.".into());
+        }
+
         if !warnings.is_empty() {
             let warnings = Formatter::new(&self.source, warnings).colored().to_string();
             warn!(message = "VRL compilation warning.", %warnings);
@@ -233,6 +237,12 @@ mod test {
                 Ok(()),
                 Ok(()),
             ),
+            (
+                log_event![],
+                r#""i_return_a_string""#,
+                Err("VRL conditions must return a boolean.".into()),
+                Ok(()),
+            ),
         ];
 
         for (event, source, build, check) in checks {