feat: add alerting for unreachable errors

changelog.d/7046-unreachable-error-alerting.added

@@ -0,0 +1 @@
+Added `error!`-level logging with `UNREACHABLE_ERROR_TRIGGERED` marker and `stacks_unreachable_errors_total` Prometheus counter when unreachable errors are hit during transaction processing ([#7046](https://github.com/stacks-network/stacks-core/pull/7046))

clarity/src/vm/analysis/errors.rs

@@ -662,9 +662,17 @@ pub struct StaticCheckError {
 
 impl RuntimeCheckErrorKind {
     /// This check indicates that the transaction should be rejected.
+    /// Currently identical to `is_unreachable()` since `Unreachable` is the only
+    /// rejectable variant, but they answer different questions and may diverge.
     pub fn rejectable(&self) -> bool {
         matches!(self, RuntimeCheckErrorKind::Unreachable(_))
     }
+
+    /// Returns true if this error is an unreachable error, indicating a potential bug.
+    /// Used only for monitoring (logging + prometheus counter), not for business logic.
+    pub fn is_unreachable(&self) -> bool {
+        matches!(self, RuntimeCheckErrorKind::Unreachable(_))
+    }
 }
 
 impl StaticCheckErrorKind {
@@ -676,6 +684,12 @@ impl StaticCheckErrorKind {
             _ => false,
         }
     }
+
+    /// Returns true if this error is an unreachable error, indicating a potential bug.
+    /// Used only for monitoring (logging + prometheus counter), not for business logic.
+    pub fn is_unreachable(&self) -> bool {
+        matches!(self, StaticCheckErrorKind::Unreachable(_))
+    }
 }
 
 impl StaticCheckError {

clarity/src/vm/ast/errors.rs

@@ -215,6 +215,15 @@ impl ParseError {
         }
     }
 
+    /// Returns true if this error is an unreachable error, indicating a potential bug.
+    /// Used only for monitoring (logging + prometheus counter), not for business logic.
+    pub fn is_unreachable(&self) -> bool {
+        matches!(
+            *self.err,
+            ParseErrorKind::InterpreterFailure | ParseErrorKind::UnexpectedParserFailure
+        )
+    }
+
     pub fn has_pre_expression(&self) -> bool {
         self.pre_expressions.is_some()
     }