revert(aso-overlay): rollback #2822 to fix dev 500 (response.headers.raw is not a function)

[abhishekgarg18]

Summary

Reverts #2822 (`feat(aso-overlay): CloudWatch EMF instrumentation for READ path`) to restore the dev overlay endpoint. Since #2822 deployed to dev (2026-07-16T03:52Z), every GET /config/dev/*/redirects.txt returns HTTP 500 with:

```
x-error: response.headers.raw is not a function
```

Verified reproduction on dev with a valid ASO_OVERLAY_API_KEY, on multiple tenants:

• `cm-p117653-e365853` (Maksym's test tenant)
• `cm-p128729-e2179647` (Okta dev)

Consistent, non-intermittent, on both the 200 and 304 conditional-GET code paths.

Timeline

Time (UTC)	Event
2026-07-15T20:23Z	Maksym verified end-to-end key rotation working on dev cm-p117653-e365853 (no `invalid X-ASO-API-Key` errors)
2026-07-16T00:03Z	Maksym verified full purge-on-Deploy chain: Deploy → S3 write → Mystique purge (`fastly_purge OK, 24ms`) → edge flipped in ~2s
2026-07-16T03:52Z	#2822 merged (v1.655.0 promoted #2822)
2026-07-16T~10:30Z	500s observed on dev overlay endpoint

Nothing else in the overlay hot path changed in this window. Other PRs merged after this window (#2820, #2833, #2804, #2823) don't touch redirects.js, aso-overlay-key-handler.js, or the metrics-emf infrastructure.

Root cause (in progress)

The error `response.headers.raw is not a function` is thrown at `@adobe/helix-universal/aws-adapter.js:254`:

```js
...splitHeaders(response.headers.raw(), con.log),
```

That helper expects `@adobe/fetch`-style Headers (which have `.raw()`). The Response returned to aws-adapter has a native Web-Fetch-API Headers instance (no `.raw()`).

Local repro confirms `createResponse` from `@adobe/spacecat-shared-http-utils` correctly returns `@adobe/fetch` Response with `.raw()` working (see snippet below). So the failure is Lambda-bundle-specific — likely an esbuild resolution/tree-shaking interaction introduced by #2822's new imports (`aso-overlay-metrics.js` constants + widened use of `metrics-emf.js`).

```js
// Local test (from within the repo)
import { createResponse } from '@adobe/spacecat-shared-http-utils';
const r = createResponse('body', 200, { 'content-type': 'text/plain' });
console.log(typeof r.headers.raw === 'function'); // true — works locally
```

Not fully root-caused yet. Reverting to unblock dev while investigation continues. This is the safest immediate fix — every branch of the overlay endpoint is failing, and the metrics that #2822 added are non-critical (dashboards can wait a day).

Follow-up plan (post-revert)

1. Bring feat(aso-overlay): CloudWatch EMF instrumentation for READ path (SITES-48140) #2822's changes back behind a functional-parity test that exercises the actual helix-deploy bundle (not just source-level tests) — mirroring the SITES-45260 pattern documented in CLAUDE.md ("works in tests, breaks in production is the bundling layer")
2. Bisect: try re-adding `aso-overlay-metrics.js` alone, then `metrics-emf.js` widening, to identify the exact bundling interaction
3. Consider whether all `emitMetric` call sites should be wrapped in an isolate-and-log outer try/catch at the wrapper level (defense-in-depth beyond the current in-function try/catch)

Impact if not reverted

• All ASO dispatcher-overlay traffic on dev returns 500 → no dispatcher can fetch overlays → broken-backlinks redirect layer non-functional on dev
• Blocks Alina's TTL bump verification (task 37)
• Blocks any stage/prod promotion (dev is the release gate)

Test plan

• Local unit tests still pass after revert (18/18 in redirects.test.js, 8/8 in aso-overlay-key-handler.test.js)
• After merge + deploy: verify dev endpoint returns 200/304/404 as expected with valid key
• Confirm Maksym's earlier end-to-end verification still passes

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[abhishekgarg18]

Superseded by #2839 — going with the forward-fix (rewrap wrapper) instead of a revert so #2822's EMF instrumentation stays live once the fix deploys.