fix(stepper): handle evaluator errors gracefully in BasicEvaluator and RunnerPlugin

[Shrey5132]

This PR introduces error handling in BasicEvaluator and RunnerPlugin to catch and handle evaluator-side errors, propagating RunnerStatus.ERROR to the host. This prevents the host REPL loop from hanging indefinitely when evaluator plugins crash or fail to initialize.

[gemini-code-assist]

Code Review

This pull request introduces error handling to the evaluator startup process by wrapping the execution loop in try-catch blocks. In BasicEvaluator.ts, the evaluator loop is now monitored to notify the host of errors, and RunnerPlugin.ts has been updated to handle these errors asynchronously. Feedback highlights that the code following the infinite loop in BasicEvaluator.ts is unreachable and may cause compilation errors. Additionally, it is recommended to log the caught error in RunnerPlugin.ts rather than swallowing it silently to aid in debugging.

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[gemini-code-assist]

This code is unreachable because of the preceding infinite while (true) loop. If allowUnreachableCode is disabled in tsconfig.json, this will also cause a compilation error. Since the loop never terminates normally, these lines should be removed.

[gemini-code-assist]

The caught error is currently swallowed silently. Swallowing errors makes debugging extremely difficult because the stack trace and error message are lost. It is highly recommended to log the error to console.error so that the failure details are visible in the logs.

            } catch (e) {
                console.error("Evaluator error:", e);
                // BasicEvaluator already sends RunnerStatus.ERROR; this is a safety net for
                // evaluator subclasses that override startEvaluator without the same guarantee.
                this.updateStatus(RunnerStatus.ERROR, true);
            }