Studio: leave a bracketed IPv6 host unchanged in dial_host - #12733

Confirmed dial_host in studio/backend/utils/host_policy.py still turns "[::1]" into "[[::1]]" on main, unlike published_url_host just above it, and this mirrors that guard. Will get this reviewed.

Pushed a small follow-up on top of the original fix and merged the latest main (the branch was 281 commits behind).

What changed:

• published_url_host and dial_host now share one private helper for the bracket rule instead of each carrying a copy. published_url_host behaves exactly as before; the two differ only in whether the zone id is percent-escaped.
• The dial_host test gained zone-id cases (fe80::1%en0 stays literal, bracketed or not), and published_url_host gained a bracketed-plus-zone-id case.

Note for reviewers: I traced both callers and no current path hands dial_host a bracketed host (the value comes from the bound socket address, the ASGI server pair, or the 127.0.0.1 fallback, and --host "[::1]" fails to bind), so this is hardening that keeps the two formatters from drifting rather than a fix for a failure users can hit.

cd studio/backend && python -m pytest tests/test_bind_host_policy.py -q   # 68 passed

Review and CI summary for head e21422e6cb, checked on a staging mirror of the same commit.

Codex review: converged — "Didn't find any major issues".

CI: 40 passed, 2 skipped, 1 failed (run). The one failure is (Python 3.13, a-k), with 7 tests in tests/test_hub_token_caller_identity.py. It is not caused by this PR: the same 7 tests fail in the same job on main at the commit this branch merged (main run), and this branch differs from that commit only in utils/host_policy.py and tests/test_bind_host_policy.py.

What changed during this pass:

• Merged the latest main (the branch was 281 commits behind, no conflicts).
• published_url_host and dial_host now share one helper for the bracket rule; published_url_host behaves as before.
• Added zone-id cases to the dial_host test and a bracketed-plus-zone-id case to the published_url_host test.

There were no unresolved review threads to address.