fix(openclaw): bound media hydration and preserve existing attachments

[changeroa]

Summary

A successful verdict POST could still leave the final reply waiting indefinitely on the image GET or body. Carry the existing request AbortSignal through hydration, and preserve existing single and multiple attachments when adding emotion media. Covers F04/F16.

Includes separate stalled-header and stalled-body regressions and a single-attachment preservation case. This PR is independent of the watcher delivery proposal; #137 touches the same entrypoint and may require a rebase when merged.

Verification

• Node 22.18.0; isolated branch checked against dev (da2120e).
• OpenClaw: 469 passed; TypeScript passed.
• Service-owned boundary and branch-flow checks passed; git diff --check passed.
• No real image generation, external channel sends, deployment, or gateway restart. Tests use fixtures, mocks, or local loopback; production HENT_AI_/DISCORD_ variables were removed from test processes.
• node scripts/release-gate.mjs — full release gate and real gateway replacement E2E were not run; no release-readiness claim.

Architecture Boundary

OpenClaw remains a thin media adapter with no new classifier, profile, asset lookup, or direct Discord REST. Changeset Validation requires owner review because openclaw/index.ts changes; no approval or owner-reviewed label is asserted.

Part of the September 8 functional audit split. Security vulnerability investigation and reproduction are outside this work. All split PRs target dev; there is no stacked branch dependency.