fix(sdk): align Python SDK with gateway dkg_evidence API + add CI

[gilbertsahumada]

Summary

• Python SDK: submit_work and submit_work_and_wait now send dkg_evidence instead of
  thread_root/evidence_root, matching the current gateway API
• CI pipeline: Added test workflow that runs gateway unit tests and Python SDK tests on every PR to main
• Test port fix: Changed test GATEWAY_URL from localhost:3000 to localhost:39999 to avoid conflicts with local
  dev servers

Context

Both SDKs (Python and TypeScript) were out of sync with the gateway API:

• SDKs sent: thread_root + evidence_root
• Gateway expects: dkg_evidence (array of evidence packages)
• Result: 400 — "dkg_evidence must be an array"

The gateway computes thread_root and evidence_root internally from the dkg_evidence array via the DKG engine. This
PR fixes the Python SDK. The TypeScript SDK (separate repo) needs a separate PR.

Test results

• Python SDK: 30 passed
• Gateway unit tests: 352 passed

Note

The TypeScript SDK (chaoschain-sdk-ts) has the same issue and needs to be updated separately.