chore(ray-image): bump version matrix to latest 3 Ray releases

[Alyssa Vu (alyssa1303)]

Summary

Bumps the images/ray build matrix (images/ray/versions.json) to append the
3 latest Ray releases published in ray-project/ray
and move the default (:latest-tagged) combo forward:

Ray version	Before	After
2.54.0	present	present
2.55.1	present	present
2.56.0	present	present
2.56.1	present (default)	present
2.57.0	—	added
2.58.0	—	added, default

images/ray/Makefile's RAY_VERSION fallback and all version references in
images/ray/README.md are synced to the new default (2.58.0).

⚠️ Update: rebased on #233, approach revised

#233 merged while this PR was open, with a different fix than my original
draft (it moved to Ray 2.56.1 upstream, not a local protobuf<5 pin), and
it also introduced a new documented policy for this matrix: append-only —
preserve historical entries, don't drop them (versions.json is "release
input, not evidence of published availability", and matrix changes must
"coordinate ... with the ADO release owner").

I merged main into this branch and resolved the conflicts to follow that
policy instead of my original plan (which replaced the matrix with only the
3 latest entries). Net effect on top of current main:

• versions.json: append 2.57.0 and 2.58.0, move default: true from
  2.56.1 to 2.58.0. All 4 pre-existing entries (2.54.0/2.55.1/2.56.0/2.56.1)
  are untouched.
• Makefile: RAY_VERSION fallback 2.56.1 → 2.58.0.
• README.md: version-specific examples/table values updated to 2.58.0.
  I also corrected one line the automatic line-based merge combined
  incorrectly — the "Image Tag Format" section's prose said "currently
  published 2.56.0" while the code block under it had been merged to show
  2.58.0; restored the code block to 2.56.0 since I have no evidence a
  2.58.0 image has actually been published (publishing is owned by the ADO
  repo per the new CI/CD section, not this PR).

Verification

• No CI workflow in this repo parses versions.json directly.
• tests/e2e/stack/harness/{fineweb,large_gpu}_ib_conformance.sh resolve
  their Ray image via jq -er 'max_by(.ray | split(".") | map(tonumber))'
  against versions.json — reran that exact query against the merged file
  and confirmed it resolves to 2.58.0 as expected.
• python3 -c "import json; json.load(open('images/ray/versions.json'))" —
  valid JSON, exactly one default: true entry (verified with
  jq '[.[] | select(.default==true)] | length' → 1).
• make -n docker-build / make -n test — Makefile renders
  RAY_VERSION=2.58.0 correctly, no syntax issues.
• python3 scripts/check-license-headers.py passes.
• Repo-wide grep for leftover <<<<<<</=======/>>>>>>> markers — none.

Scope note

Still scoped to the images/ray build matrix only — not the other repo
references to the currently-published ray:py3.12-ray2.56.0-cuda13.0 tag
(examples/, charts/taugrid-core, tests/e2e fixtures, cluster-overlays
dashboards). Per the new README, bumping what's actually published/consumed
is an ADO-owned release step, not a contributor PR.

Test plan

• ADO release build of images/ray for the new matrix entries
  (2.57.0, 2.58.0), per the repo's ADO-owned build/publish process.
• make test against each newly built image.

[Pengfei Ni (feiskyer)]

LGTM

[Kevin Cho (chokevin)]

some pending comments offline but the idea looks good to me