test: bump default integration test image from focal to resolute

[nryanl]

Proposed Commit Message

test: bump default integration test image from focal to resolute

Update the default OS_IMAGE for integration tests to resolute (26.04),
the most recent LTS, and update the comment demonstrating its internal
image specification.

Additional Context

Test Steps

Verified tox -e integration-tests -- tests/integration_tests/test_paths.py runs with OS_IMAGE=resolute locally after change.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)