ci(bulk-export): add external smoke workflow

[aacruzgon]

Summary

Implemented the Bulk Export external smoke test plan.

Added:

• .github/workflows/bulk-export-smoke.yml

What it does

• Manual-only workflow_dispatch.
• Builds HFS once with R4,R4B,R5,sqlite,elasticsearch,postgres,mongodb,s3.
• Runs a 36-job matrix across FHIR versions, primary backends, and bulk export modes.
• Full lifecycle smoke runs for currently bulk-wired sqlite and postgres.
• Expected-negative rows cover current unsupported/unwired surfaces like MongoDB, S3 primary, and composite Elasticsearch modes.
• Captures HTTP artifacts, manifests, NDJSON downloads, summaries, HFS logs, and backend logs.

Test plan

• Trigger the workflow manually from the Actions tab
• Verify the matrix fan-out produces 36 jobs
• Confirm sqlite and postgres lifecycle jobs pass end-to-end
• Confirm expected-negative rows exit with the anticipated failure codes
• Check uploaded artifacts contain logs, manifests, and NDJSON files