fix(adapters): set explicit text/plain content type on blocked responses

[rennf93]

Summary

• Blocked/error responses built by DjangoResponseFactory.create_response fell back to Django's default text/html; charset=utf-8. Under X-Content-Type-Options: nosniff (applied by default), that is exactly the gap fastapi-guard fixed in #144: the block page should be explicit text/plain.
• create_response now passes content_type="text/plain; charset=utf-8". Redirect responses are untouched (no body, no content type).
• Behavior beyond the default is preserved: custom_error_responses text stays text/plain, and a custom_response_modifier can still set its own content type.

Tests

• New tests/test_block_response_content_type.py, mirroring fastapi-guard's test of the same name: block response is plain text with nosniff, custom error message stays plain text, modifier override wins.
• Extended test_django_response_factory_create with the header assertion.
• Full suite locally: 278 passed.

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