These files were not touched by the recent performance and
correctness PRs but had coverage gaps. Adds tests for:
- CLI: serve, queue init/migrate, config loading, cli() entry point
including MigrationRequiredError exit path
- filter: _default_supported_extensions, __call__ watchfiles callback,
FileFilter with supported_extensions=None
- registry: resolve_adhoc_fetcher with bucket-less S3 URI
- migrations: pragma no-cover on unreachable schema upgrade path
(no diff migrations exist until SCHEMA_VERSION > 1)
