fix(searchapi): add null-safe parsing to SearchApiAPIWrapper and introduce defensive tests

[MANYA12-STAR]

Description

This PR improves the robustness of the SearchApiAPIWrapper by adding null-safe parsing, defensive checks, and a more resilient _result_as_string() extraction flow.

Previously, missing or partially-filled fields returned by SearchAPI (e.g., missing answer_box, absent snippet, empty knowledge_graph, or sparse organic_results) could produce KeyError, inconsistent behavior across engines, or unexpected empty output.

This update introduces:

• Safe dictionary access via .get() patterns
• Graceful fallback ordering across all supported response fields
• Defensive handling for missing or malformed items in arrays
• Consistent string assembly for organic_results, jobs, videos, and images
• Improved reliability for both sync and async flows

As a result, the wrapper now behaves predictably across different SearchAPI engines and response variations, avoiding runtime errors while still returning meaningful textual output.

Issue

Closes #338 — Improve SearchApiAPIWrapper to support missing fields and prevent runtime parsing errors.

Dependencies

• No new dependencies introduced
• No changes to pyproject.toml
• All imports remain optional where appropriate