GeezerVison

A configured remote with no URL currently either fails lookup or returns a
NULL URL. Git uses the remote's name in both cases. Follow that behavior,
including URL rewrites and an explicit push URL, while still returning
GIT_ENOTFOUND when the remote has no configuration.

Recognize remotes with settings other than URL/pushURL in lookup and listing.
This also means creating a remote over an existing prune-only configuration
returns GIT_EEXISTS, as Git does. Update the prune test to load its configured
remote instead of recreating it.

Reject a bare URL/pushURL setting with no value instead of confusing it with
an explicit empty value. Add lookup, rewrite, list, local connection and
rename/set-url/delete coverage; document the fallback.

Fixes #7028

Validation:

• Built current Git 1891de46 and checked missing/empty URL, push-only,
  rewrite, explicit pushURL and missing-remote behavior.
• Focused remote suites: 57 tests pass (network::remote::remotes 40,
  remote::list 2, network::fetchlocal 15).
• Native Debug offline selection: 3073 pass, 44 skip, 0 fail.
• GCC 11.4 -Werror with ASan/UBSan and leak detection: 7 remote suites
  (125 tests) clean; offline selection 3073 pass, 43 skip, 0 fail, 0 sanitizer
  reports.
• Offline selection excludes online and clone::nonetwork. Both builds exclude
  core::structinit, whose padding comparison fails on unchanged baseline.
  Sanitizer selection additionally excludes pack::threadsafety: the unexcluded
  run reports 8848 bytes / 75 allocations leaked, identically reproduced on
  baseline.
• Linux x86_64 only; HTTPS/SSH/NTLM unavailable and disabled.

Known differences from Git:

• A bare, value-less url/pushURL entry makes lookup fail with a generic
  invalid-argument error where Git exits with a config error; both reject it.
• git_remote_list still lists a remote whose url entry has no value; Git's
  git remote fails the whole config read in that case.
• A multi-valued url keeps last-value-wins (pre-existing behavior, unchanged).