feat: add toBeEmail expectation (#1735)

Adds `toBeEmail()` backed by `filter_var(FILTER_VALIDATE_EMAIL)`.
Mirrors the existing `toBeUrl()` pattern — no new dependencies, pure PHP.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Mixins/Expectation.php

@@ -1171,6 +1171,22 @@ final class Expectation
         return $this;
     }
 
+    /**
+     * Asserts that the value is an email address.
+     *
+     * @return self<TValue>
+     */
+    public function toBeEmail(string $message = ''): self
+    {
+        if ($message === '') {
+            $message = "Failed asserting that {$this->value} is an email address.";
+        }
+
+        Assert::assertTrue(Str::isEmail((string) $this->value), $message);
+
+        return $this;
+    }
+
     /**
      * Asserts that the value is a url
      *