fix(rlhf): percent-encode the IPv6 zone id in rollout base URLs

[Redemption-ZTX]

What

format_host_for_url wraps IPv6 addresses in brackets but copies the address verbatim, so a scoped address produces a URL with an unencoded zone id:

format_host_for_url('fe80::1%eth0')  # -> '[fe80::1%eth0]'   (before)
                                     # -> '[fe80::1%25eth0]'  (after)

RFC 6874 requires the zone id to be percent-encoded inside the brackets. Left unencoded, the URL carries %et, which HTTP clients reject as a malformed percent-escape.

Where it matters

format_host_for_url has a single call site, building the rollout base URLs from the caller-supplied hosts:

self.base_urls = [f'http://{format_host_for_url(h)}:{p}'
                  for h, p in zip(hosts, server_ports)]   # vllm_client.py:100

Any scoped (link-local) IPv6 reaching that list — e.g. from a config that addresses a node by its link-local address — yields a URL that HTTP clients reject.

Correction: an earlier version of this description claimed the address comes from resolve_hostname() (vllm_client.py:91). That is wrong: resolve_hostname() sits in the other branch of the same if/else, and its result only feeds self.hosts (used for bind_host and the weight-sync connection at :239 and :253), never format_host_for_url. The helper itself is still wrong on scoped input, which is what this PR fixes, but the reachability is narrower than I first wrote.

Change

Only the single % separator is encoded (host.partition('%')), so a raw scoped address is encoded exactly once. Addresses without a zone id are unaffected, and ipaddress.IPv6Address rejects the bracketed form, so an already-wrapped host is not wrapped twice.

resolve_hostname and is_valid_ipv6_address are left untouched.

Tests

Adds tests/general/test_ipv6_host_format.py (9 cases): IPv4 and hostname pass-through, plain IPv6 bracketing, zone-id encoding, the already-bracketed form, and that the built URL still parses into the expected port and path.

python -m pytest tests/general/test_ipv6_host_format.py -q

• 9 passed with the fix
• Verified the tests catch the bug: reverting format_host_for_url to the original body fails test_scoped_ipv6_zone_is_percent_encoded
• pre-commit clean (flake8, isort, yapf)

No GPU required — this is pure ipaddress / urllib.parse logic.