fix: EML file decoding crash due to UnicodeDecodeError

[onkar0127]

fixes #149

Description

This Pull Request resolves the crash that occurred when uploading non-UTF-8 encoded .eml files (e.g. accented characters in Latin-1 / ISO-8859-1) to the Sender Verifier tool.

Changes

1. Resilient Fallback Decoding (backend/api.py):
   • Updated the /analyze-email-header endpoint to attempt strict UTF-8 decoding first.
   • If a UnicodeDecodeError is caught, it falls back to decoding via latin-1 with errors="replace". This ensures legacy or mixed-encoding files are read successfully without raising decode exceptions.
2. Added Unit Tests (backend/tests/test_email_header_analyzer.py):
   • Added a new unit test case test_api_endpoint_multipart_eml_non_utf8 that simulates uploading a Latin-1 encoded .eml file containing accented characters (ñ) and verifies it resolves successfully.

Verification Results

• Pytest: Executed pytest backend/tests and all 59 tests passed with zero failures.

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