test : added unit tests for classify_memory_violation and resolve_sandbox_config

[tmdeveloper007]

Closes #1224.

Summary of What Has Been Done:
Created testing/backend/unit/test_sandbox_executor.py covering the classify_memory_violation and resolve_sandbox_config pure helpers from backend.secuscan.sandbox_executor. These are not covered by testing/backend/test_sandbox_executor.py which tests sandbox_execute end-to-end.

Changes Made:

• classify_memory_violation: 10 tests covering exit-code detection (-11 SIGSEGV, 139 Linux SIGSEGV, exit 0, non-zero), stderr text detection (MemoryError, Cannot allocate memory), and RSS threshold detection (95%%, 96%%, below 95%%, ignored on exit 0)
• resolve_sandbox_config: 3 tests covering no-override (returns global defaults), partial override (only overridden fields change), and full override (all fields match plugin config)
• Settings patching uses config_module.settings attribute swap with try/finally for isolation
• 14 tests total, all passing

Impact it Made:
classify_memory_violation is a post-mortem heuristic for OOM kills. Incorrect classification could misattribute memory-limit exits. resolve_sandbox_config is the gate between global and per-plugin sandboxing. Both are now unit-tested for regression safety.

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