kobotoolbox / kpi / 28865799815

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fix(submissions): reject submissions with invalid xml character DEV-1129 (#6385)

### 📣 Summary
Reject form submissions containing invalid or invisible XML characters,
to prevent data corruption when responses are later viewed or edited.


### 📖 Description
Some clients can submit XML content containing characters outside the
range permitted by the XML standard (invisible control characters,
unpaired surrogates, etc.). These submissions could previously be stored
as-is, only to later fail when opened or edited in Enketo, risking data
loss or broken records.

This adds server-side validation (validate_xml_chars) that rejects such
submissions upfront with a clear 400 error message, instead of letting
them silently corrupt data down the line. Only characters permitted by
the XML standard are accepted.

This validation is kept as a safety net for any client (not just Enketo)
that might still submit disallowed characters, now that Enketo itself
has been fixed to strip them before submission.

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