ci: add conformance test workflow

[sbeashwar]

Summary

• Adds a GitHub Actions workflow that runs the full conformance test suite
• Checks out conformance, samples, and SDK repos in correct relative paths
• Starts the Flower Shop reference server with test data
• Runs all *_test.py files and reports per-file results using GitHub log groups
• Triggers on: PRs to main, pushes to main, nightly schedule, manual dispatch
• Results are informational (continue-on-error) - does not block merges

This would have caught the breakage from #28 before merge (see #37).

Addresses #38

Test plan

• Verify workflow runs on this PR
• Confirm server health check wait loop works
• Confirm all test files are discovered and executed

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[sbeashwar]

@cusell-google gentle ping - this is a small, focused change ready for review when you have time.

[cusell-google]

Thanks again @sbeashwar for the nice, small incremental PRs!

[sbeashwar]

Verified end-to-end on my fork: https://github.com/sbeashwar/conformance/actions/runs/24915126403

All 13 test files pass (59/59). The red status on the run is from a post-job uv cache prune cleanup timeout (known setup-uv cache lock issue, unrelated to the workflow itself) — the Run conformance tests step succeeded.

[sbeashwar]

On a second thought, updated to disable post-job uv cache prune (was timing out on a lock and turning the run red even though tests passed). Latest run is fully green: https://github.com/sbeashwar/conformance/actions/runs/24915690807

[sbeashwar]

@cusell-google addressed the two review comments, switched to python-sdk and gated continue-on-error to PR runs only. Also disabled post-job uv cache prune so the verification run goes fully green: https://github.com/sbeashwar/conformance/actions/runs/24915690807. Ready for re-review.

[sbeashwar]

@cusell-google addressed the review feedback in the latest commits. Test plan verified on the fork:

• Workflow runs on this PR
• Server health check wait loop works
• All test files are discovered and executed

Ready for re-review when you have a moment.

[cusell-google]

Thanks!