Added ContentTypeOwnedBySomeoneElseException for content type draft owned by another user

[bnowak]

🎫 Issue	IBX-XXXXX

Description

ContentTypeService::loadContentTypeDraft() threw a generic NotFoundException with the message
'The content type is owned by someone else' when a draft existed but was owned by a different user,
carrying a long-standing @todo Use another exception when user of draft is someone else.

Consumers had no way to distinguish that ownership case from a genuinely missing draft other than by
matching the exception message, which is fragile — the message can change here and silently break
those consumers.

This PR introduces a dedicated Ibexa\Core\Repository\ContentType\Exception\ContentTypeOwnedBySomeoneElseException
(extending Ibexa\Core\Base\Exceptions\NotFoundException) and throws it from loadContentTypeDraft(),
resolving the @todo. Callers can now branch on the exception type via instanceof.

The exception passes the same 'The content type is owned by someone else' description to its parent,
so the resulting message is unchanged and existing catch (NotFoundException $e) blocks keep working.

Motivation

Raised in review of the MCP get_content_type_draft tool:
https://github.com/ibexa/mcp/pull/14#discussion_r3466283486

[alongosz]

I'd prefer to see some minimal test coverage for the new exception, but +1.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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