fix: add debug_assert invariant guards to critical code paths (#1312)

* fix: add debug_assert invariant guards to critical code paths (closes #1215)

Add three debug_assert! calls to catch impossible-in-correct-code states
early in debug builds without affecting release performance:

- execute_tool_with_safety: assert tool_name is non-empty at entry
- JobContext::transition_to: assert state machine transition is valid
- CircuitBreakerProvider::record_success: assert circuit is not Open
  (check_allowed() must gate all calls before record_success())

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* test: add regression test for empty tool name invariant guard

Covers the debug_assert!(!tool_name.is_empty()) added in execute_tool_with_safety.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

src/context/state.rs

@@ -258,6 +258,13 @@ impl JobContext {
         new_state: JobState,
         reason: Option<String>,
     ) -> Result<(), String> {
+        debug_assert!(
+            self.state.can_transition_to(new_state),
+            "BUG: invalid job state transition {} -> {} for job {}",
+            self.state,
+            new_state,
+            self.job_id
+        );
         if !self.state.can_transition_to(new_state) {
             return Err(format!(
                 "Cannot transition from {} to {}",