Contact phone numbers with stray text in front of them, like
`abc+12312312321`, were saving successfully instead of being rejected as
invalid. Agents could end up with unusable numbers on a contact, and the
same values were persisted rather than discarded when captured through
the live chat widget.
## How to reproduce
1. Open a contact and edit its details.
2. Set the phone number to `abc+12312312321` via the API (`PATCH
/api/v1/accounts/:id/contacts/:id`).
3. Before this change the update succeeds. Now it fails validation.
## What changed
The E.164 format check was missing a leading `\A` anchor, so Rails
matched it anywhere in the string and accepted any prefix ahead of a
valid number. Both the validation and the `phone_number_format` fallback
used by `discard_invalid_attrs` are now anchored, so the widget path
discards these values instead of storing them.
Contacts already holding a prefixed number will now fail validation on
their next save. Worth a count on production first:
```sql
SELECT count(*) FROM contacts WHERE phone_number !~ '^\+[1-9][0-9]{1,14}$' AND phone_number <> '';
```
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