This PR introduces a safe mechanism for parent workflows to signal their children through two new features:

1. ChildWorkflowHandle Class

A newChildWorkflowHandle class that wraps a child workflow and provides context-safe signaling:

• Context Preservation: Automatically saves and restores parent workflow context when forwarding signals to children
• Replay Safety: Skips re-signaling during workflow replay to avoid duplicate signals
• Simple API: Uses__call() magic method to forward any signal method to the child

2. Workflow Methods:child() andchildren()

Two new methods added to the baseWorkflow class:

• child(): Returns aChildWorkflowHandle for the most recently created child workflow at the current execution index
• children(): Returns an array ofChildWorkflowHandle instances for all child workflows at the current execution index

Both methods aredeterministic - they filter children byparent_index <= $this->index, ensuring consistent results during workflow replay.

Usage Examples

Example 1: Parent Directly Signals Child

publicfunctionexecute(){// Start child (don't yield yet)$childPromise = ChildWorkflowStub::make(ChildWorkflow::class,'arg');// Get handle and signal child$childHandle =$this->child();$childHandle->approve('approved');// Wait for child to complete$result =yield$childPromise;return$result;}

Example 2: Parent Forwards External Signal to Child

class ParentWorkflowextends Workflow{privatebool$receivedSignal =false;private ?string$signalData =null;    #[SignalMethod]publicfunctionexternalSignal(string$data):void    {$this->receivedSignal =true;$this->signalData =$data;    }publicfunctionexecute()    {// Start child$childPromise = ChildWorkflowStub::make(ChildWorkflow::class);// Get child handle (called once in execute, saved to local variable)$childHandle =$this->child();// Wait for external signalyield WorkflowStub::await(fn () =>$this->receivedSignal);// Forward signal to child using saved handleif ($childHandle &&$this->signalData) {$childHandle->processData($this->signalData);        }// Wait for child completion$result =yield$childPromise;return$result;    }}