fix(core): add fail-fast validation to SiftBuilder.shake()

- Validate the generated regex string using Pattern.compile().

- Throw a descriptive IllegalStateException if the pattern is malformed.

- Add testShakeFailFastValidation to verify exception handling.

Author: Mirkoddd

sift-core/src/main/java/com/mirkoddd/sift/core/SiftBuilder.java

@@ -326,6 +326,13 @@ public SiftPattern andNothingElse() {
     public String shake() {
         if (cachedRegex == null) {
             cachedRegex = assembler.build();
+
+            try {
+                java.util.regex.Pattern.compile(cachedRegex);
+            } catch (java.util.regex.PatternSyntaxException e) {
+                throw new IllegalStateException("Sift generated an invalid regex pattern: " + cachedRegex +
+                        ". Please report this bug to the library maintainers.", e);
+            }
         }
         return cachedRegex;
     }

sift-core/src/test/java/com/mirkoddd/sift/core/SiftTest.java

@@ -437,6 +437,7 @@ void testBuilderIdentity() {
 
             assertNotEquals(builderA, "^[a-zA-Z]??", "A builder should not be equal to an object of a different class");
         }
+
         @Test
         @DisplayName("toString() should return the generated pattern string")
         void testBuilderToString() {
@@ -448,6 +449,23 @@ void testBuilderToString() {
             assertEquals(builder.shake(), builder.toString(),
                     "toString() must rely on the cached shake() output");
         }
+
+        @Test
+        @DisplayName("shake() should perform fail-fast validation and throw IllegalStateException on invalid regex")
+        void testShakeFailFastValidation() {
+            SiftPattern malformedPattern = () -> "(?unclosedGroup";
+
+            com.mirkoddd.sift.core.SiftBuilder builder =
+                    (com.mirkoddd.sift.core.SiftBuilder) Sift.fromStart().pattern(malformedPattern);
+
+            IllegalStateException exception = assertThrows(IllegalStateException.class, builder::shake,
+                    "shake() should throw an IllegalStateException if the generated regex is physically invalid");
+
+            assertTrue(exception.getMessage().contains("Sift generated an invalid regex pattern"),
+                    "The exception message should clearly indicate a Sift compilation error");
+            assertInstanceOf(java.util.regex.PatternSyntaxException.class, exception.getCause(),
+                    "The root cause must be the original PatternSyntaxException from java.util.regex");
+        }
     }
 
     @Nested