Add CHANGELOG for 2.7.0

[gchan]

Description

Rename ## Unreleased to ## 2.7.0 - 2026-05-28 and add the four other user-visible changes that have landed on main since v2.6.0:

• Opt-in test result upload to the Upload API (Upload native test runner output to Upload API from bktec #519, Upload pytest test results to Upload API #527).
• Pytest default output switched to JUnit XML when buildkite-test-collector is absent (Change pytest runner default output to JUnit; keep test-collector JSON as an option #524).
• JUnit XML parser now treats <error> elements as failures (Handle <error> elements in JUnit XML parser #526).
• cwd included in run_env for uploaded results, so the receiving side can resolve absolute paths back to repo-relative ([TE-5972] Include cwd in run_env when uploading test results #522).

Decisions Made

Minor version bump to 2.7.0, not patch to 2.6.1. The original release-cut sequence for TE-5984 anticipated a 2.6.x patch carrying only the OIDC fallback extension. PRs #519 and #527 add a new opt-in feature (test result upload), and #524 changes default behaviour for the Pytest runner. Semver puts both in minor-bump territory.

Bundle the rest of main into this release. The alternative is to cut 2.7.0 with only the upload work and follow up with 2.7.1 for the OIDC fallback. Bundling minimises the number of bktec versions downstream consumers (notably test-prediction-backfill, which TE-5860 will pin) need to consider, and the upload work is already verified on main.

Context

Resolves TE-5984's release-cut step. This is step 3 of the release-cut sequence: land #523, then this, then cut v2.7.0 from the release pipeline.

This PR is stacked on top of #523. Merge #523 first; this PR will then auto-retarget to main and the diff reduces to the four extra bullets and the Unreleased → 2.7.0 - 2026-05-28 heading rename.

Verification

Diff against #523 is CHANGELOG-only — visually reviewable. The release pipeline is what actually validates the new 2.7.0 heading.

AI assisted.

Rollback

Yes.