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/util.py

@@ -5,9 +5,18 @@ level deeper, under their backend package."""
 
 from __future__ import annotations
 
+import ipaddress
 import os
 
 
+def is_ip_literal(value: str) -> bool:
+    try:
+        ipaddress.ip_address(value)
+    except ValueError:
+        return False
+    return True
+
+
 def expand_tilde(path: str) -> str:
     """Expand a leading '~' to $HOME. Leaves paths without a leading
     tilde unchanged. Falls back to the empty string if $HOME is unset