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.
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