test(backend): cover crawler response-size limits without Content-Length

[utksh1]

Summary

Add a streaming-response test that enforces the configured size cap when Content-Length is absent.

Scope

• Suggested file: backend/secuscan/crawler.py
• Add or extend the matching backend unit or integration test.
• Keep the change focused on the documented edge case.

Acceptance criteria

• The regression case is reproducible in an automated test.
• The expected safe behavior is asserted explicitly.
• Existing behavior remains covered.
• The focused test passes locally.

Verification

Run the affected pytest file and any directly related suite.

[Sneha-Anand1910]

Hi @utksh1, I'd like to work on this issue under GSSoC '26. Could you please assign it to me?