feat: replace ADAPTIVE_EMPTY_RESULT with ReplayChildren flag

[ParidelPooya]

ReplayChildren Implementation for Run-in-Child-Context Handler

This change replaces the previous ADAPTIVE_EMPTY_RESULT marker approach with ReplayChildren flag mechanism for handling large payloads in child context operations.

Key Changes

1. ChildConfig Interface Enhancement

• Added summaryGenerator?: (result: T) => string option
• summaryGenerator Will be used internall to create a summary for ctx.map and ctx.parallel when result is big
• Note: summaryGenerator receives the raw result (before serialization)

2. Checkpoint Behavior Update

• Before: Used magic string "__LARGE_PAYLOAD__" as payload marker
• After: Uses ContextOptions: { ReplayChildren: true } flag with empty string payload (or custom summary)

3. Replay Detection Logic

• Before: if (result === ADAPTIVE_EMPTY_RESULT)
• After: if (stepData?.ContextDetails?.ReplayChildren)

4. Data Flow Details

• Size Check: Uses serialized result (after serdes.serialize()) to determine if payload > 256KB
• summaryGenerator: Receives raw result (before serialization) for flexible summary creation

Workflow Diagram

┌─────────────────────────────────────────────────────────────────────────────┐
│                    Child Context Execution Flow                             │
└─────────────────────────────────────────────────────────────────────────────┘

                              ┌─────────────────┐
                              │  Execute Child  │
                              │    Context      │
                              └─────────┬───────┘
                                        │
                              ┌─────────▼───────┐
                              │ Raw Result (T)  │
                              │                 │
                              └─────────┬───────┘
                                        │
                              ┌─────────▼───────┐
                              │ Apply Serdes    │
                              │ Serialization   │
                              │ result → string │
                              └─────────┬───────┘
                                        │
                              ┌─────────▼───────┐
                              │ Check Serialized│
                              │ Payload Size    │
                              │ > 256KB?        │
                              └─────┬───────┬───┘
                                    │       │
                               NO   │       │ YES
                                    │       │
                    ┌───────────────▼─┐   ┌─▼──────────────────┐
                    │ Normal          │   │ Large Payload      │
                    │ Checkpoint      │   │ Handling           │
                    │                 │   │                    │
                    │ Payload:        │   │ ReplayChildren:    │
                    │ serialized data │   │ true               │
                    │ ReplayChildren: │   │                    │
                    │ undefined       │   │                    │
                    └───────────────┬─┘   └─┬──────────────────┘
                                    │       │
                                    │   ┌───▼──────────────────┐
                                    │   │ summaryGenerator     │
                                    │   │ provided?            │
                                    │   │ (gets RAW result)    │
                                    │   └───┬──────────┬───────┘
                                    │       │          │
                                    │   YES │          │ NO
                                    │       │          │
                                    │   ┌───▼────┐ ┌───▼────┐
                                    │   │Custom  │ │Empty   │
                                    │   │Summary │ │String  │
                                    │   │from    │ │""      │
                                    │   │raw data│ │        │
                                    │   └───┬────┘ └───┬────┘
                                    │       │          │
                                    └───────┼──────────┘
                                            │
                              ┌─────────────▼─────────────┐
                              │     Checkpoint Saved      │
                              └─────────────┬─────────────┘
                                            │
                              ┌─────────────▼─────────────┐
                              │      On Replay...         │
                              └─────────────┬─────────────┘
                                            │
                              ┌─────────────▼─────────────┐
                              │ Check ReplayChildren      │
                              │ flag in ContextDetails    │
                              └─────┬───────────┬─────────┘
                                    │           │
                               FALSE│           │TRUE
                                    │           │
                    ┌───────────────▼─┐   ┌─────▼──────────────┐
                    │ Return Cached   │   │ Re-execute Child   │
                    │ Result from     │   │ Context Function   │
                    │ Checkpoint      │   │ (Reconstruct       │
                    │ (deserialize)   │   │ Full Result)       │
                    └─────────────────┘   └────────────────────┘

Data Flow Summary

1. Execute child context function → Raw result T
2. Serialize result using serdes → string
3. Check size of serialized string
4. If large: Call summaryGenerator(rawResult) → custom summary
5. Checkpoint with ReplayChildren flag and summary/empty payload
6. On replay: Check ReplayChildren flag to decide re-execution

Checklist

• [Y] I have filled out every section of the PR template
• [Y] I have thoroughly tested this change

Testing

Unit Tests

Yes

[yaythomas]