Use VuMarkService in VuMark generation API tests

[adamtheturtle]

This updates VuMark generation API tests to use vws-python's VuMarkService and VuMarkAccept for client-level behavior checks. It converts the empty instance ID and non-VuMark target tests to assert typed vws exceptions (InvalidInstanceIdError and InvalidTargetTypeError) and inspect their responses. It keeps raw signed requests where direct protocol/header control is required and adds a dedicated test for response Content-Type across PNG/SVG/PDF formats. The PR diff is limited to tests/mock_vws/test_vumark_generation_api.py.

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Note

Low Risk
Test-only changes that primarily refactor assertions and add a header check; no production logic or security-sensitive code is modified.

Overview
Updates VuMark generation API tests to exercise the vws client surface: generation now uses VuMarkService.generate_vumark_instance with typed VuMarkAccept values, and the format test validates returned bytes by file signature.

Error-path coverage is shifted from raw HTTP assertions to asserting InvalidInstanceIdError/InvalidTargetTypeError exceptions and inspecting their embedded HTTP responses. Raw signed requests are retained where direct header control is needed, with a new dedicated test that validates the Content-Type header for PNG/SVG/PDF responses.

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