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/llm/circuit_breaker.rs

@@ -167,6 +167,12 @@ impl CircuitBreakerProvider {
                 }
             }
             CircuitState::Open => {
+                debug_assert!(
+                    false,
+                    "BUG: record_success() called while circuit breaker is Open — \
+                     check_allowed() was bypassed for provider {}",
+                    self.inner.model_name()
+                );
                 // Shouldn't get here (check_allowed blocks Open), but recover
                 state.state = CircuitState::Closed;
                 state.consecutive_failures = 0;