fix: remove debug_assert guards that panic on valid error paths (#1385)

* fix: remove debug_assert guards that panic on valid error paths (#1312)

Two debug_assert! calls added in #1312 fire on expected runtime error
paths (not programmer bugs), turning graceful error returns into panics
in debug/test builds:

- state.rs: Completed→Cancelled is a user-facing error handled by
  transition_to() returning Err — not a bug
- execute.rs: empty tool_name from malformed LLM output is handled by
  ToolError::NotFound — not a bug

Removes both asserts; keeps the circuit-breaker assert (genuinely guards
a caller invariant).

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix: tighten empty tool name test to assert ToolError::NotFound variant

Address review feedback: assert the specific error variant instead of
just is_err() so the regression test actually enforces the expected
error path.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* style: cargo fmt

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/context/state.rs

@@ -258,13 +258,6 @@ 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 {}",