test: restore callback metadata contract

[chubes4]

Summary

• restore the exact callback materialization fixtures for bounded preview-proxy response metadata
• assert normalized content length, service-worker root scope, and body rewrite semantics directly
• retain exact object matching for redirect and truncated-response trace records

Root cause and contract decision

d4a734f8 intentionally added contentLength, serviceWorkerAllowed, and bodyRewritten to the typed bounded request trace for packaged-worker diagnostics. b86524a0 subsequently relied on the service-worker scope metadata, while adjacent browser tests assert rewrite behavior. The callback materialization fixture retained the older exact shape, so the fixture rather than the owning contract was stale.

The response metadata remains bounded optional contract output. This PR updates exact expectations rather than weakening them to partial matching.

Verification

• npm run test:browser-callback-materialization-contracts passes
• npm run test:service-worker-preview-proxy passes, 4/4
• npm run test:playground-contained-scope passes, 1/1
• npm run build passes
• npm run check passes the repaired callback contract, then stops at the independent pre-existing command-registry-smoke fails on main: wordpress.collect-workload-result outputShape missing outputSchema id mention #1745 command-registry-smoke failure: wordpress.collect-workload-result outputShape should mention outputSchema id. Fresh reproduction recorded at command-registry-smoke fails on main: wordpress.collect-workload-result outputShape missing outputSchema id mention #1745 (comment)

AI assistance

• AI assistance: Yes
• Tool: OpenCode
• Used for: contract investigation, test fixture updates, verification, and PR preparation

Fixes #2344