Test Run IDs tie parallel runs together

Set DONOBU_RUN_ID once in CI and every flow produced by that run, across parallel shards and workers, gets grouped under a single identifiable test run.

Flows now carry an optional run ID, sourced from the DONOBU_RUN_ID environment variable. Set it once in a CI step (a GitHub Actions run ID, a custom value, anything stable for that run and distinct from others) and every flow produced by that test run, including parallel shards and workers, gets grouped together under one identifier.

This shows up as a single row in the Test Runs view, rather than a scatter of individually timestamped flows you have to reconstruct into a run by hand. Runs that don't set the variable behave exactly as before.