test : add unit tests for _build_triage_prompt in triage_engine

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the _build_triage_prompt() helper function in backend/secuscan/triage_engine.py. This private function constructs the prompt string passed to the LLM when triaging a SAST finding. It formats the finding title, category, severity, description, code context, and variables into a structured prompt.

The existing test file testing/backend/unit/test_triage_engine.py tests extract_code_context, is_eligible_for_triage, triage_finding, and triage_findings, but _build_triage_prompt is not directly imported or tested.

Changes that Need to be Made:
In testing/backend/unit/test_triage_engine.py:

• Import _build_triage_prompt from backend.secuscan.triage_engine
• Test that the prompt includes the finding title
• Test that the prompt includes the finding category
• Test that the prompt includes the severity
• Test that the prompt includes the description
• Test that the prompt includes the code snippet from context
• Test that the prompt includes variable hints
• Test that missing/None values fall back to defaults like "Untitled finding", "(no source snippet available)", "(none identified)"
• Test that the prompt ends with the response format instructions (JSON object requirements)
• Test that the prompt is a pure function (same inputs always produce same output)

Impact that it would Provide:

• Directly tests the prompt construction logic without needing a mock LLM
• Ensures the LLM receives well-formed prompts with all required fields
• Catches regressions if the prompt template changes

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