fix(manifest): actionable errors when USCOGDATA_URL is unset or returns non-JSON

`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.

R/session.R

@@ -5,6 +5,7 @@
 #' @noRd
 cog_open <- function(url = .resolve_url(),
                      cache_dir = .resolve_cache_dir()) {
+  .check_url_configured(url)
   if (!dir.exists(cache_dir)) dir.create(cache_dir, recursive = TRUE)
 
   con <- DBI::dbConnect(duckdb::duckdb())