fix: harden report template for missing fields

[AlwinXue]

Hi mentors! 👋

This PR addresses the PDF generation crash reported in Issue #38.

The Problem:
The render_report_html function used strict dictionary lookups and assumed a single data shape for asl_parameters. If the processor output omitted keys like basic_report or extended_report, or if asl_parameters arrived as a dictionary instead of a list of tuples, the PDF endpoint crashed before it could render a response.

The Solution:
Safe Lookups: Replaced strict indexing with .get() calls and added sensible fallback strings for missing report sections.
Format Flexibility: Added support for asl_parameters as either a dictionary or an iterable of (key, value) pairs.
Key Fallbacks: The template now checks missing_required_parameters first and falls back to missing_parameters for older payloads.
Regression Coverage: Added a targeted backend test to verify the PDF endpoint still works when missing_required_parameters is provided and report text fields are absent.

Validation:
Ran python -m pytest tests/test_report.py -q -k "test_report_pdf_endpoint or test_report_pdf_endpoint_uses_missing_required_parameters" (with DYLD_FALLBACK_LIBRARY_PATH=/opt/homebrew/lib) and both targeted PDF tests passed.

Fixes #38