bug: fix prototype pollution in chat message handler

[devin-ai-integration]

Summary

PUT /chat merged the attacker-controlled req.body.message straight into a fresh object with _.merge, so {"message": {"__proto__": {"canDelete": true}}} polluted Object.prototype and granted every user delete rights (see exploits/prototype-pollution.sh c3/c4).

chat.add now builds the message from an explicit field allowlist instead of merging:

for (const field of ['icon', 'text']) {
  if (input && Object.prototype.hasOwnProperty.call(input, field)) { /* must be a string */ }
}

__proto__/constructor are never own-enumerable keys that get copied, and non-object or non-string input is rejected with 400. chat.delete additionally requires canDelete to be an own property equal to true, so any other pollution vector cannot grant the capability.

Devin-Org: engineering

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