feat: show messages for URL result exporters

[CenturySturgeon]

Summary

• Allow URL result exporters to return an optional structured result with url, message and message_type.
• Display the exporter message in the existing "export complete" modal while retaining the user clicked "View Export" action.
• Preserve compatibility with exporters that return a plain URL string.

Test plan

• make test
• Verified structured URL results show a warning and open the supplied URL only after clicking "View Export".
• Verified legacy plain string URL results still use the existing "export complete" modal flow.

Compatibility

Existing URL exporters may continue returning a plain string URL unchanged. The structured result is opt-in:

{
    "url": "https://example.com/export",
    "message": "Optional warning for the user",
    "message_type": "warning",
}

[raymondtieu]

lgtm