test : added unit tests for routes JSON deserialization helpers

[tmdeveloper007]

Closes #1065.

Summary of What Has Been Done:
Added testing/backend/unit/test_routes_json_helpers.py covering the three pure helper functions exported by backend.secuscan.routes_json_helpers. The module was extracted from routes.py in a prior merge; these tests directly import and exercise the real production functions. 14 tests, all passing.

Changes Made:

• Created testing/backend/unit/test_routes_json_helpers.py
• parse_json_fields: parses JSON string fields, preserves non-string/falsy fields, keeps original string on decode error, does not mutate input
• deserialize_finding_rows: strips *_json suffix and renames to friendly keys (metadata_json -> metadata), partial fields only rename present keys, does not mutate input
• deserialize_asset_service_rows: parses and renames metadata_json and cert_san_json, ignores unknown *_json fields, does not mutate input
• FINDING_JSON_FIELDS constant: verified to contain the expected six field names

Impact it Made:
Zero test coverage for the routes JSON deserialization helpers. These functions run on every finding and asset-service row returned from the database; untested, a regression in the JSON parsing or renaming logic could silently corrupt or drop finding data in the API responses.

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.