fix: handle form validation errors more robustly in document number extraction

[seer-by-sentry]

👋 Hi there! This PR was automatically generated by Autofix 🤖

This fix was triggered by Michael Lissner

Fixes DOCTOR-27

This commit improves the error handling in the document number extraction view.

• Switches from request.GET to request.POST for form data.
• Improves error message extraction from the form, handling cases where the error is in __all__, file, or other fields.
• Adds a test case to verify that form validation errors are handled correctly.

If you have any questions or feedback for the Sentry team about this fix, please email [email protected] with the Run ID: 10129.

[sentry]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: doctor/views.py

Function	Unhandled Issue
get_document_number	KeyError: 'all' /utils/document-number/pdf/ Event Count: 1

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[mlissner]

@flooie, can you take a look at this one, please? Sentry generated a fix for a bug it detected, and then created this pull request (!). It might be garbage, but the error was real, and we should fix it. Looks fairly straightforward, I think!