bundle_builder_x: --all-channels override validation picks stale/oldest base revision for multi-base channels

While validating the `canonical-livepatch-server-k8s.yaml` override fix in #961 with
`--all-channels`, `test_charm_override_file_is_valid[canonical-livepatch-server-k8s[beta]]`
and `[edge]` failed with:

```
UnparsableCharmException: Charm 'canonical-livepatch-server-k8s' override declares
config keys not present in charm config at channel beta: ['ingress-interface']
```

## Root cause

Charmhub's `latest/beta` channel for this charm maps to three different
revisions, one per supported base:

```
beta ubuntu@20.04 rev 49
beta ubuntu@22.04 rev 89 <- current/actual latest
beta ubuntu@24.04 rev 50
```

rev 49 is a much older revision that predates the `ingress-interface` config
option entirely (confirmed via
`canonical/livepatch-k8s-operator` tag `rev49` vs `rev89` `config.yaml`).
`bundle_builder_x.charmhub.CharmhubClient` resolves the channel to base 20.04
(`_get_ubuntu_version_from_bases`: "Return first ubuntu version with matching
base") instead of the base actually intended/used by the rest of this
project's deployments, so validation checks config/endpoint keys against a
stale revision.

## Impact

Any override that adds a `configs:` entry and validates with `--all-channels`
against a charm whose channel spans multiple bases with divergent
revisions/config schemas can produce false failures unrelated to the override
itself.

Note: the default (non-`--all-channels`) validation used by
`scripts/bundle-builder-x-tests.sh overrides` is unaffected, since it only
checks one (the first-matched) channel per override criterion.

## Suggested fix

When resolving a channel with `--all-channels` (or generally, wherever a
requested base is not explicit), prefer the newest/highest base rather than
the first one encountered, or make base selection configurable/consistent with
how the actual test-plan runs select platform/base.

SQT-1178

Start in bundle_builder_x/charmhub.py at CharmhubClient and _get_ubuntu_version_from_bases, then reproduce the beta and edge failures with test_charm_override_file_is_valid and --all-channels. Trace how the selected base and revision are chosen; done means multi-base channels validate against the intended newest or configurable base without breaking existing override tests.

Written by the indexing model from the issue text.