fix(mcp): require the metadata release carrying the APIs the component uses

[Amoifr]

PHPUnit api-platform/mcp (PHP 8.5 lowest) fails on every pull request targeting 4.3, including #8525 and #8530, with 11 errors:

Error: Call to undefined method ApiPlatform\Metadata\McpTool::getTitle()
Error: Class "ApiPlatform\Metadata\Exception\BadRequestException" not found

The component allows api-platform/metadata: ^4.3, so composer update --prefer-lowest resolves v4.3.0, which carries neither API:

Used	Where	Released in
McpTool::getTitle()	Capability/Registry/Loader.php:66	v4.3.5
Metadata\Exception\BadRequestException	Tests/Server/HandlerTest.php:345	v4.3.19

The second one arrived with #8525, which fixed the previous round of this on the same job, so the floor simply trailed behind the code twice.

Raising it to ^4.3.19 matches what the other components already do, api-platform/serializer:^4.3.12 and api-platform/validator:^4.3.1.

Verified by running what the workflow runs, in src/Mcp:

composer update --prefer-lowest   # v4.3.0  -> Tests: 80, Errors: 11
composer update --prefer-lowest   # v4.3.19 -> OK (80 tests, 248 assertions)

4.4 and main are unaffected, they require ^4.4 and ^5.0.