[TE-6280] Move Playwright tagging from tag to annotation with buildkite.tag. prefix

[nprizal]

Right now we tag Playwright test executions using Playwright's native tag field, and we only parse tags matching the @tag_key:value syntax. This isn't a great fit because Playwright's tag is closer conceptually to a Test Engine test label than a tag. Test Engine tags are key/value pairs, and Playwright's annotation field is a better match for that.

We made the same call for JUnit already, where we only ingest tags with a buildkite.tag. prefix. This brings Playwright in line with that approach.

• Parses annotation entries with a buildkite.tag. prefix and sends them to Test Engine as tags, e.g. buildkite.tag.team: "my-team" uploads as team: "my-team".
• Keeps the old @key:value tag syntax working for existing customers, with a deprecation warning printed once per run.
• Updates the README to document the new annotation-based tagging and mark the old syntax as deprecated.

[buildsworth-bk-app]

No findings from my pass. Because this changes Playwright reporter behavior rather than an L1-only change, and I don’t see an L2 approval grant, I’m commenting rather than approving.

Want to dig deeper? The full session log is attached to this Buildkite build. Download the session file and open a new pi session with it:

Download the buildsworth logs from build 2580, then answer my questions about the findings.