fix(whatsapp_identity): pin identifier regex to ASCII, clarify it's defense-in-depth

Follow-up on top of #16243. Two small tweaks:

- Compile the regex once as `_SAFE_IDENTIFIER_RE` and pin it to
  `[A-Za-z0-9@.+\-]`. The previous `\w` accepts Unicode word chars
  (full-width digits, accented letters) which aren't valid WhatsApp
  identifiers and shouldn't reach the mapping-file lookup.
- Add a comment clarifying this is defense-in-depth, not a live
  traversal. The hardcoded `lid-mapping-{current}{suffix}.json`
  prefix already prevents escape via pathlib's component split —
  with `current='../secrets'`, the first path component under
  `session/` is the literal directory name `lid-mapping-..`,
  which the attacker cannot create.

E2E verified: legit mapping chains still resolve, all probed attack
shapes (`../`, absolute paths, shell metacharacters, Unicode digit
tricks) are rejected before any file access.

gateway/whatsapp_identity.py

@@ -37,6 +37,11 @@ from typing import Set
 
 logger = logging.getLogger(__name__)
 
+# WhatsApp JIDs are numeric (or plus-prefixed numeric) with optional
+# ``@``, ``.`` and ``:`` separators. ``\w`` is pinned to ASCII so
+# full-width digits / Unicode word chars can't sneak through.
+_SAFE_IDENTIFIER_RE = re.compile(r"^[A-Za-z0-9@.+\-]+$")
+
 from hermes_constants import get_hermes_home
 
 
@@ -85,7 +90,15 @@ def expand_whatsapp_aliases(identifier: str) -> Set[str]:
         current = queue.pop(0)
         if not current or current in resolved:
             continue
-        if not re.match(r'^[\w@.+-]+$', current):
+        # Defense-in-depth: reject identifiers that could sneak path
+        # separators / traversal segments into the ``lid-mapping-{current}``
+        # filename below. The hardcoded ``lid-mapping-`` prefix already
+        # prevents escape via pathlib's component split (an attacker can't
+        # create ``lid-mapping-..`` as a real directory in session_dir), but
+        # this keeps the identifier space to the characters WhatsApp JIDs
+        # actually use and avoids depending on that filesystem-layout
+        # invariant.
+        if not _SAFE_IDENTIFIER_RE.match(current):
             continue
 
         resolved.add(current)