Replaces String-typed build strings on the core records with the
dedicated newtype:

- PackageRecord::build, IndexJson::build, MinimalPrefixRecord::build,
  GenericVirtualPackage::build_string: String -> BuildString.

HasArtifactIdentificationRefs::build now returns &BuildString; lockfile
v5/v6/v7 conda models and v7 source models store Cow<'a, BuildString>
instead of Cow<'a, str>.

PackageRecord::new takes a BuildString directly so callers pick the
strictness explicitly via BuildString::strict / ::unchecked. All call
sites across rattler, rattler-bin, rattler_cache, rattler_index,
rattler_lock, rattler_repodata_gateway, rattler_solve,
rattler_virtual_packages, py-rattler and js-rattler are updated to
match.

…entry points

Address the second round of review feedback that arrived after the first
refactor commit was pushed.

- BuildString::append/prepend now also reject empty results, restoring
  the [1, 64]-byte invariant for combined values. Adds BuildStringError::Empty.
- Drop the unused append_unchecked/prepend_unchecked variants; production
  code only ever needs the validating versions.
- py-rattler PackageRecord(...) constructor (PyRecord::create) now returns
  PyResult and raises ValueError on invalid build strings, matching the
  set_build setter.
- js-rattler PackageRecord JSON constructor and the simple_solve locked-
  packages path validate build strings via BuildString::new instead of
  going through the permissive serde / new_unchecked entry points.
- Test fixtures that depended on the permissive behaviour migrated from
  py3-none-any / cp39-cp39-linux_x86_64 to underscore-separated build strings.

Makes "BuildString may be empty" expressible, but then it was via the
Option before.