fix(mcp): fix ignored output format - #8542

StructuredContentProcessor serializes a tool result with

    $format = $request->getRequestFormat('') ?: 'jsonld';

and never reads $operation->getOutputFormats(), although $operation is in scope
and FormatsResourceMetadataCollectionFactory has populated it. The MCP route
carries no _format placeholder or default either, so getRequestFormat() is always
empty and the expression is a constant 'jsonld' on every call.

McpFormatTool declares outputFormats: ['json' => ['application/json']]. The first
test asserts the metadata honours it (passes); the two others assert the payload
and structuredContent honour it (both fail, showing the JSON-LD envelope).

Declaring the format on the operation exercises the same code path as
api_platform.mcp.format, which is implemented as withOutputFormats() on the
operation -- so this single reproducer covers both ways of configuring it.

Two CI failures, both mine.

PHPStan: process() receives the base Operation type, which does not declare
getOutputFormats() -- only HttpOperation does. The lookup now sits behind the
same McpTool/McpResource check the structuredContent flag already used, hoisted
into a local so both read it.

PHPUnit: the MongoDB test app loads routing_mongodb.yml, the only environment
that does not import routing_test.php and therefore has no /mcp route, so the
tests answered 404 instead of skipping. McpTest guards on isMongoDB() for the
same reason; McpFormatTest now does too, via RecreateSchemaTrait.