test: replace SASSERT with ENSURE, remove Windows-only guards (#10086)

Unit tests relied on `SASSERT()` which is a no-op in release builds
(`DEBUG_CODE` wrapper), silently skipping all assertions outside debug
mode. Several test files were also gated behind `#ifdef _WINDOWS`,
making them dead code on Linux/macOS CI.

## Changes

- **`SASSERT` → `ENSURE` in 20 test files (200 occurrences)**: `ENSURE`
maps to `VERIFY` and always executes regardless of build type, ensuring
test assertions are active in both debug and release builds.

- **`src/test/diff_logic.cpp`**: Removed `#ifdef _WINDOWS` wrapping the
entire file. No Windows-specific APIs were used; the guard only
prevented compilation on non-Windows platforms.

- **`src/test/dl_product_relation.cpp`**: Removed `#ifdef _WINDOWS`
guard around `tst_dl_product_relation()`. The function body has no
platform dependencies.

- **`src/test/sat_local_search.cpp`**: Replaced `sscanf_s`
(MSVC-specific) with portable `sscanf`; added return-value check to
detect malformed input. Previously, `build_instance()` unconditionally
returned `false` on non-Windows, making the SAT local search test a
no-op on Linux/macOS.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/test/dl_product_relation.cpp

@@ -351,7 +351,6 @@ namespace datalog {
 
 using namespace datalog;
 
-#ifdef _WINDOWS
 void tst_dl_product_relation() {
     smt_params fparams;
     params_ref params;
@@ -362,7 +361,3 @@ void tst_dl_product_relation() {
     test_finite_product_relation(fparams, params);
     
 }
-#else
-void tst_dl_product_relation() {}
-
-#endif