fix: Add deterministic error clearing for FAIL_AFTER_EXECUTABLE_NODES_COMPLETE policy

[andreahlert]

Summary

This PR addresses the determinism concern raised by @hamersaw in #4624 regarding error clearing behavior when multiple nodes fail with FAIL_AFTER_EXECUTABLE_NODES_COMPLETE policy.

The Problem

When multiple nodes fail, we want to preserve only one error to reduce etcd state size. The original implementation in #4624 would clear errors during iteration, but didn't account for the fact that Go map iteration order is non-deterministic. This could cause ALL errors to be cleared if iteration order varied between reconciliation rounds:

1. Round 1: Iterate [node-a, node-b] - clear node-a's error, keep node-b's
2. Round 2: Iterate [node-b, node-a] - node-b has error, clear it, then node-a has no error anymore

The Solution

Add a nil check before considering a node as having a "new" failure:

if nodeStatus.GetExecutionError() != nil {
    if nodeStatusWithError != nil && !c.enableCRDebugMetadata {
        nodeStatusWithError.ClearExecutionError()
    }
    nodeStatusWithError = nodeStatus
}

Once an error is cleared, that node won't be re-processed in subsequent rounds, ensuring determinism regardless of iteration order.

Changes

• Add ClearExecutionError() method to MutableNodeStatus interface
• Implement ClearExecutionError() in NodeStatus struct
• Add nil check in handleDownstream before error clearing
• Add comprehensive unit tests validating the deterministic behavior

Test Plan

• Unit tests for ClearExecutionError() method
• Test that only the last error is preserved when multiple failures occur
• Test that clearing already-nil errors is a no-op
• All existing node controller tests pass

Related Issues

• Fixes part of [Core feature] Reduce unnecessary workflow state stored on etcd to avoid hitting etcd size limits #4569 (Reduce etcd state for large workflows)
• Addresses determinism concern in Clear past errors from workflow state #4624

[welcome]

Thank you for opening this pull request! 🙌

These tips will help get your PR across the finish line:

• Most of the repos have a PR template; if not, fill it out to the best of your knowledge.
• Sign off your commits (Reference: DCO Guide).

[andreahlert]

Hi @hamersaw, friendly ping! This addresses the determinism concern you raised in #4624 regarding error clearing with non-deterministic map iteration. CI is green - would love your review when you get a chance. Thanks!