test: use isolated RandomState in linear algebra tests

Convert remaining np.random.randn calls to use isolated RandomState:
- np.dot.Test.cs: Dot30_300x30_300 test
- MatMulInt64Tests.cs: Dot_LargeContiguousArrays test

Ensures test isolation for parallel execution.

Author: Nucs

test/NumSharp.UnitTest/LinearAlgebra/MatMulInt64Tests.cs

@@ -247,8 +247,9 @@ public async Task MatMul_Int64_Correct()
     public async Task Dot_LargeContiguousArrays()
     {
         // Test with larger arrays to ensure SIMD path handles size correctly
-        var a = np.random.randn(100, 100);
-        var b = np.random.randn(100, 100);
+        var rng = np.random.RandomState(42);
+        var a = rng.randn(100, 100);
+        var b = rng.randn(100, 100);
 
         // Should not throw
         var result = np.dot(a, b);

test/NumSharp.UnitTest/LinearAlgebra/np.dot.Test.cs

@@ -101,8 +101,9 @@ public void Dot2x3And3x2()
         [OpenBugs] // RNG IndexOutOfRangeException in MT19937
         public void Dot30_300x30_300()
         {
-            var a = np.random.randn(30, 300);
-            var b = np.random.randn(300, 30);
+            var rng = np.random.RandomState(42);
+            var a = rng.randn(30, 300);
+            var b = rng.randn(300, 30);
             var c = np.dot(a, b);
         }
     }