[hoanle0126]

Closes #524
Supersedes #526 (rebased cleanly onto latest main after #522/#527).

Summary

When a screenshot probe reaches a cloud grid after the session has already ended, the hub can respond with HTTP 404 and a JSON body such as {"value": {"message": "Session not started or terminated"}} (or {"message": "..."} with no error field). Previously:

1. packages/core/src/webdriver-http.ts did not reject non-2xx status codes and isWebdriverError only checked typeof value.error === 'string', returning the error object as if it were a base64 screenshot string.
2. That object flowed through captureActionSnapshot, upsertRichestSnapshot, and FrameSnapshotIndex into imageDocument, where imageMime called .startsWith on the object and aborted the entire trace.zip export.

This PR implements both the transport fix and defence-in-depth guards across snapshot capture, screencast polling, and trace export:

• packages/core/src/webdriver-http.ts: Reject non-2xx HTTP responses before parsing value, reject bodies without a value property, and treat value objects carrying a string message with no payload keys as errors in isWebdriverError.
• packages/core/src/action-snapshot.ts: Keep screenshot only when shot is a non-empty string.
• packages/core/src/screencast.ts: Guard #takeFirstShot, the #startPolling interval, and pushCdpFrame so only non-empty strings are appended to the frame buffer.
• packages/trace/src/trace-frame-snapshots.ts, trace-snapshots.ts, trace-exporter.ts: Skip non-string or empty screenshot values in FrameSnapshotIndex, imageDocument, buildImageFrameSnapshots, upsertRichestSnapshot, buildSnapshotResources, and synthesizeSnapshotsFromCommands so a malformed frame falls back to the preceding valid frame rather than failing the zip export.

Verification

• npx vitest run packages/core/tests/webdriver-http.test.ts packages/core/tests/action-snapshot.test.ts packages/core/tests/screencast.test.ts packages/trace/tests/trace-frame-snapshots.test.ts packages/trace/tests/trace-exporter.test.ts packages/trace/tests/trace-snapshots.test.ts (119 passed)
• pnpm --filter @wdio/devtools-core --filter @wdio/devtools-trace lint
• npx tsc --noEmit -p tsconfig.json