External API access to support bundle filters

[smklein]

Stacked on #11113.

BundleDataSelection describes what a support bundle collects: which
categories, which sleds to query, and the bundle-wide time window that bounds
zone logs and ereports. omdb support-bundle collect could already drive that
collection, but Nexus hardcoded BundleDataSelection::all() at bundle creation,
so none of it was reachable over the API.

This adds external API version SUPPORT_BUNDLE_DATA_SELECTION, which
gives SupportBundleCreate an optional data_selection and returns the
stored selection from the view endpoint as a new SupportBundleView.
Omitting data_selection preserves the previous behavior: collect
everything within the last 7 days.

POST /v1/system/support-bundles
{
  "user_comment": "disk errors on BRM42",
  "data_selection": {
    "data": {
      "type": "explicit",
      "reconfigurator": true,
      "host_info": { "sleds": { "type": "specific", "sleds": ["..."] } },
      "ereports": { "only_classes": ["hw.pwr.*"] }
    },
    "start_time": "2026-08-20T00:00:00Z"
  }
}

A couple things worth calling out:

"Collect everything" is an explicit variant, so an empty category selection
unambiguously means "collect none of these" instead of being a rejected
special case. Sled selection then mirrors the internal SledSelection enum
one to one.

omdb nexus support-bundles create grows the --include, --since, and
--until flags that support-bundle collect already had. The
age-to-timestamp handling behind --since and --until is now shared
between the two commands.

Test coverage

The first commit adds unit tests for the conversions between the API type
and BundleDataSelection, including a proptest asserting that every
stored selection survives a round trip through the API type, and cases
pinning what all and an empty explicit selection map to.

The second commit covers the API end to end. A bundle created without a
data selection collects every category and carries the seven-day start
bound Nexus stamps, matching the behavior before the selection was
exposed; asking for everything explicitly is equivalent. An explicit
selection round-trips: a category subset, specific sleds, ereport serial
and class filters, and an explicit window all come back from the view as
they were sent, and an explicit selection naming nothing creates a bundle
that collects nothing. Creation rejects an inverted window and a sled
that does not exist.

One case is about older bundles: viewing a bundle whose time range row is
absent reports no bounds rather than failing. Bundles collected before
time ranges existed have no such row, because #11113's migration only
creates rows for bundles still awaiting collection. The category rows are
not affected, since those were backfilled when their tables landed.