fix(client): keep status and body when a 422 is not a validation error

[Ashfaqbs]

What

DoclingServeClient.getResponse parses every 422 body as a ValidationError. If the body is not one (for example an HTML error page from a gateway or proxy in front of docling-serve), the parse failure escapes as a raw Jackson exception, and the status code and response body are lost.

Change

If the 422 body cannot be parsed as a validation error, fall back to DoclingServeClientException carrying the status code and body, the same as any other 4xx/5xx response. Real validation errors still throw ValidationException.

Tests

New DoclingServeClientErrorResponseTests (WireMock, no container needed):

• a 422 with a validation body still throws ValidationException
• a 422 with an HTML body throws DoclingServeClientException with status 422 and the body

Before the change the second test fails with tools.jackson.core.exc.StreamReadException; after it, both pass. spotlessCheck reports no violations in the files touched here.

Signed off per DCO.