Fix bulk edit validation: prevent duplicate findings from being active/verified

[valentijnscholten]

Description

This PR fixes the bug where bulk edit allows duplicate findings to be marked as active/verified, which violates business rules enforced in the regular edit form and API. And it adds other validation rules that were missing from bulk edit.
AI helped me to create test cases. And then I extracted logic into separate methods to please Ruff.

Fixes #11336

Changes

Form Validation ()

• Added validation in FindingBulkUpdateForm.clean() to prevent active findings from being risk accepted
• This matches the validation present in FindingForm but was missing from the bulk form

View Validation ()

• Added view-level validation to check existing duplicate status before setting active/verified
• Added view-level validation to check existing active status before risk accepting
• Added comprehensive user feedback messages for skipped findings with specific reasons:
  • Warning when duplicate findings are skipped during status updates
  • Warning when active findings are skipped during risk acceptance
• Updated success message to use actually_updated_count to accurately reflect findings that were actually updated (excluding skipped ones)

Validation Rules Enforced

All validation rules from the API and regular forms are now enforced in bulk edit:

1. ✅ Duplicate findings cannot be active/verified
2. ✅ False positive findings cannot be verified
3. ✅ Active findings cannot be risk accepted
4. ✅ Risk acceptance requires product setting enabled

User Feedback

Users now receive clear feedback about:

• How many findings were successfully updated
• How many findings were skipped and why (duplicate status, active status, authorization, etc.)

[mtesauro]

Approved