Skip task creation for pre-flight failures (payment, validation)

[jirispilka]

Problem

When paymentRequiredResult is set (missing payment signature), we still create a task and pass the error into executeToolAndUpdateTask(), which immediately stores it as a completed-with-error result. The task serves no purpose — we already know the outcome before execution.

Same applies to any future pre-flight check that can be resolved synchronously.

Current flow

prepareToolCallContext() → paymentRequiredResult set
AJV validation → pass (payment fields stripped)
task created ← unnecessary
setImmediate → executeToolAndUpdateTask → immediately stores error result

Proposed fix

Return the paymentRequiredResult synchronously (like the non-task path at line ~818), skip task creation entirely. The client gets the x402 payload faster and no zombie task is created.

if (request.params.task && paymentRequiredResult) {
    // No task needed — return payment error synchronously
    return paymentRequiredResult;
}

Context

Discovered during #680 (task status workaround). See res/task_status_workaround.md for the full SDK limitation analysis.