refactor: address PR review feedback — de-privatize helpers and rename modules

- Rename _manifest_util.py → manifest_util.py (module isn't private)
- Rename _as_json_object → as_json_object, _parse_git_upstream → parse_git_upstream,
  _parse_git_gate_config → parse_git_gate_config,
  _validate_unique_git_names → validate_unique_git_names,
  _validate_egress_routes → validate_egress_routes (none are private at
  module boundary — underscore prefix was a carry-over from the old
  monolithic manifest.py where everything lived in one namespace)
- Move _is_ip_literal → util.is_ip_literal (generic, belongs in the
  top-level util module)
- Update all import sites across manifest_*.py, manifest_extends.py,
  manifest_schema.py; existing callers of manifest.py are unaffected

All 867 unit tests pass.

bot_bottle/manifest_schema.py

@@ -58,7 +58,7 @@ def _validate_frontmatter_keys(
     keys: object,
     allowed_keys: frozenset[str],
 ) -> None:
-    from ._manifest_util import ManifestError
+    from .manifest_util import ManifestError
 
     key_set = set(keys)
     unknown = key_set - allowed_keys