Improve retry prompt with structured failure context

[Nehal-cell]

Problem

The current retry prompt sends the full test failure output to the LLM, which is noisy and wastes tokens.

Solution

Parse the pytest failure output into a structured format before sending to the LLM:

{
  "failed_tests": ["test_auth_header", "test_route_404"],
  "error_types": ["AttributeError: 'Request' has no attribute 'headers'"],
  "likely_cause": "FastAPI request object access pattern differs from Flask"
}

This gives the LLM cleaner signal and should improve retry success rate.