`cog_gov_search()` (and every other verb) used to fail with a cryptic
`jsonlite` lexical error when the package's placeholder default URL was
hit and the server returned an HTML welcome page that got cached as
`manifest.json`. Three guards added:
1. `.check_url_configured()` aborts with class `uscogdata_url_not_configured`
when the resolved URL is empty or still contains the
`REPLACE_WITH_SHARE_TOKEN` sentinel. Message names both
`Sys.setenv(USCOGDATA_URL = ...)` and `options(uscogdata.url = ...)`
remediations and points at the bundled fixture.
2. `.fetch_or_cache_manifest()` parses the response body before persisting
it. Non-JSON payloads raise class `uscogdata_invalid_manifest` (URL,
Content-Type, parse error) and never touch the on-disk cache.
3. Cache writes are atomic via a sibling tempfile + `file.rename`, and
existing caches with non-JSON content are silently refetched instead
of returning a parse error to the caller.
Local-path manifests that aren't valid JSON now surface the same
`uscogdata_invalid_manifest` class with file context.
