fix: restore the response envelope Amplitude's production parser reads

[gagantrivedi]

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Reverts the wrong half of #8400. Staging verification shows Amplitude applies the portal-configured ID path (response.list_id) on both sides, but to different objects: the Testing tab wraps our body in a {"response": ...} envelope first, while the production worker reads the raw body. The create response therefore carries the ID both flat and nested:

{"list_id": X, "response": {"list_id": X}}

The camelCase listId copy from #8399 was never read by either side and stays gone.

How did you test this code?

Unit tests assert both copies. Verified against staging: the production sync failed with only listId present and succeeded when the nested response.list_id copy existed.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Aug 28, 2026 12:06pm
flagsmith-frontend-preview	Ignored	Preview	Aug 28, 2026 12:06pm
flagsmith-frontend-staging	Ignored	Preview	Aug 28, 2026 12:06pm

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: ba43d638-f332-4dec-9251-60b1d1ee564f

📥 Commits

Reviewing files that changed from the base of the PR and between a84ba89 and dcd30fb.

📒 Files selected for processing (1)

• openapi.yaml

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📝 Walkthrough

Walkthrough

The Amplitude create response schema replaces listId with list_id at the root and inside response. The endpoint returns the cohort UUID in both locations. The OpenAPI schema defines both fields as required. Unit tests use the renamed field and verify both values. The observability documentation updates the source line for the rejected webhook event.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to dcd30

This localized change restores the response fields required by Amplitude and adds matching test coverage; no actionable merge-blocking risk remains after normal checks and review.

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-8402	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-8402	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-8402	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-8402	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-8402	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-8402	Finished ✅	Results ✅

[github-actions]

✅ private-cloud · depot-ubuntu-latest-arm-16 — run #19920 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 35.9 seconds
 a84ba89
 🔄 Run: #19920 (attempt 1)

🗂️ Previous results

✅ private-cloud · depot-ubuntu-latest-16 — run #19920 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 4 passed

Details

 4 tests across 4 suites
 10.5 seconds
 a84ba89
 🔄 Run: #19920 (attempt 1)

✅ oss · depot-ubuntu-latest-arm-16 — run #19920 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 39.7 seconds
 a84ba89
 🔄 Run: #19920 (attempt 1)

✅ oss · depot-ubuntu-latest-arm-16 — run #19921 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 40 seconds
 dcd30fb
 🔄 Run: #19921 (attempt 1)

✅ oss · depot-ubuntu-latest-16 — run #19920 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 41.5 seconds
 a84ba89
 🔄 Run: #19920 (attempt 1)

✅ oss · depot-ubuntu-latest-16 — run #19921 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 41.3 seconds
 dcd30fb
 🔄 Run: #19921 (attempt 1)

[github-actions]

Visual Regression

19 screenshots compared. See report for details.
View full report

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.81%. Comparing base (e252a36) to head (dcd30fb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8402   +/-   ##
=======================================
  Coverage   98.81%   98.81%           
=======================================
  Files        1621     1621           
  Lines       66102    66104    +2     
=======================================
+ Hits        65318    65320    +2     
  Misses        784      784           

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