fix(stepfunctions): properly deprecate DistributedMap resultWriter property

[pahud]

Issue # (if applicable)

Closes #35966.

Reason for this change

The getResultWriter() method in DistributedMap gates on the @aws-cdk/aws-stepfunctions:useDistributedMapResultWriterV2 feature flag to decide which writer to return. This causes two problems:

1. When the flag is enabled but only resultWriter is provided → returns undefined (silent data loss)
2. When the flag fails to propagate (e.g. inside a CDK Stage) → resultWriterV2 is never selected, even when explicitly provided

As reported by @aihaddad in #35966 (comment), FeatureFlags.of(this).isEnabled() returns false inside a Stage even when the flag is set to true.

Evidence

Official AWS docs confirm the expected ASL output — ResultWriter (Map) specifies that ResultWriter must contain Resource + Parameters (JSONPath) or Resource + Arguments (JSONata) to export results to S3. The CDK bug causes this entire block to be silently omitted.

Description of changes

Removed the feature flag gate from getResultWriter() and replaced it with a simple fallback:

private getResultWriter(): ResultWriterV2 | ResultWriter | undefined {
  return this.resultWriterV2 ?? this.resultWriter;
}

• resultWriterV2 is always preferred when provided
• Falls back to resultWriter for backward compatibility
• Emits a deprecation warning (not error) when resultWriter is used
• No feature flag dependency — works in all contexts (Stack, Stage, Pipeline)

How it works

Before (broken):

┌─────────────────────┐     ┌──────────────────────────┐
│ User provides        │ ──> │ getResultWriter()        │
│ resultWriter or      │     │ checks FeatureFlags      │
│ resultWriterV2       │     └──────────────────────────┘
└─────────────────────┘               │
                              ┌───────┴───────┐
                              │               │
                           flag=true      flag=false
                              │               │
                              v               v
                     ┌──────────────┐  ┌──────────────┐
                     │ return       │  │ return       │
                     │ resultWriter │  │ resultWriter │
                     │ V2           │  │ (deprecated) │
                     └──────────────┘  └──────────────┘
                              │               │
                              v               v
                     ┌──────────────┐  ┌──────────────┐
                     │ undefined if │  │ undefined if │
                     │ user only    │  │ user only    │
                     │ set V1  ✗   │  │ set V2  ✗   │
                     └──────────────┘  └──────────────┘


After (fixed):

┌─────────────────────┐     ┌──────────────────────────┐
│ User provides        │ ──> │ getResultWriter()        │
│ resultWriter or      │     │ resultWriterV2 ??        │
│ resultWriterV2       │     │ resultWriter             │
└─────────────────────┘     └──────────────────────────┘
                                       │
                               ┌───────┴───────┐
                               │               │
                          V2 provided?     V1 provided?
                            (yes)            (yes)
                               │               │
                               v               v
                      ┌──────────────┐  ┌──────────────┐
                      │ use V2  ✓   │  │ use V1  ✓   │
                      │              │  │ + warning    │
                      └──────────────┘  └──────────────┘

Before (broken):

// resultWriterV2 silently ignored inside a Stage
new DistributedMap(stack, "Map", {
  resultWriterV2: new ResultWriterV2({ bucket, prefix: "results/" }),
});
// → No ResultWriter in CloudFormation template

After (fixed):

// Works everywhere, no feature flag needed
new DistributedMap(stack, "Map", {
  resultWriterV2: new ResultWriterV2({ bucket, prefix: "results/" }),
});
// → ResultWriter correctly rendered

// Deprecated property still works with warning
new DistributedMap(stack, "Map", {
  resultWriter: new ResultWriter({ bucket, prefix: "results/" }),
});
// → ResultWriter rendered + deprecation warning emitted

Describe any new or updated permissions being added

N/A

Description of how you validated changes

• All 65 distributed-map unit tests pass
• Tests updated to verify:
  • resultWriterV2 renders regardless of feature flag state
  • resultWriter renders with deprecation warning
  • resultWriterV2 takes precedence when both are provided
• Integration tests updated to use resultWriterV2

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	144 ran	144 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	144 ran	144 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates