fix(mcp): split nullable array type into anyOf branches for wider client compatibility

[ousamabenyounes]

The MCP JSON Schema flattener now rewrites an array type (e.g. the ["string","null"] produced for nullable properties) into anyOf branches, each with a single type, while preserving sibling keywords. Several MCP clients read type as a single string and would otherwise reject the tool or drop the constraint. Base JsonSchema generation is untouched.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

PHPUnit 12.5.35 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.4.14
Configuration: /app/phpunit.xml.dist

�[41;37mF�[0m                                                                   1 / 1 (100%)

Time: 00:00.003, Memory: 18.00 MB

There was 1 failure:

1) ApiPlatform\Mcp\Tests\JsonSchema\SchemaFactoryTest::testArrayTypeIsSplitIntoAnyOf
the array `type` must be removed
Failed asserting that an array does not have the key 'type'.

/app/src/Mcp/Tests/JsonSchema/SchemaFactoryTest.php:515

�[37;41mFAILURES!�[0m
�[37;41mTests: 1�[0m�[37;41m, Assertions: 1�[0m�[37;41m, Failures: 1�[0m�[37;41m.�[0m

With the fix applied, the test passes (GREEN):

GREEN attempt 1/2 (exit 0)
PHPUnit 12.5.35 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.4.14
Configuration: /app/phpunit.xml.dist

�[33;1mN�[0m                                                                   1 / 1 (100%)

Time: 00:00.002, Memory: 16.00 MB

�[30;43mOK, but there were issues!�[0m
�[30;43mTests: 1�[0m�[30;43m, Assertions: 3�[0m�[30;43m, PHPUnit Notices: 1�[0m�[30;43m.�[0m

Full local suite

Command: docker run --rm -u "$(id -u):$(id -g)" -e HOME=/tmp -v "$PWD":/app -w /app composer:2.8 vendor/bin/phpunit --bootstrap vendor/autoload.php src/Mcp/Tests

PHPUnit 12.5.35 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.4.14
Configuration: /app/phpunit.xml.dist

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Time: 00:00.035, Memory: 20.00 MB

�[30;43mOK, but there were issues!�[0m
�[30;43mTests: 44�[0m�[30;43m, Assertions: 112�[0m�[30;43m, PHPUnit Notices: 44�[0m�[30;43m.�[0m

Fix #8504