[Capability] Stop a union type losing a branch (#434)

* [Capability] Stop a union type losing a branch

mapPhpTypeToJsonSchemaType() tested for array syntax before splitting the
union, so string[]|int mapped to array alone; applyArrayConstraints() then
overwrote the type with array whenever the union contained one, dropping the
scalar branch a second time. A tool declaring such a parameter rejected a
valid integer argument.

The split now happens first and only at the top level, so array<int|string>
and array{a: int|string} stay one type rather than two that do not exist, and
the array branch keeps its items alongside the others - which is what 2020-12
means by items: a constraint that applies only when the instance is an array.

* Apply suggestion from @chr-hertel

* Isolate the array branch before inferring its element type

* Don't apply items to arrays whose union branches disagree on element type

tests/Unit/Capability/Discovery/UnionFixture.php

@@ -0,0 +1,27 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Unit\Capability\Discovery;
+
+class UnionFixture
+{
+    /**
+     * @param string[]|int   $mixedish          an array branch beside a scalar one
+     * @param int|string     $scalars           two scalar branches
+     * @param string[]       $arrayOnly         one branch, and it is an array
+     * @param string[]|null  $nullableArray     an array branch that may be null
+     * @param string[]|int[] $disagreeingArrays two array branches with different element types
+     */
+    public function handle(array|int $mixedish, int|string $scalars, array $arrayOnly, ?array $nullableArray, array $disagreeingArrays): string
+    {
+        return '';
+    }
+}