chore: cap eval Vertex AI dependency below 2

[UniversePeak]

Link to Issue or Description of Change

• Related: chore: eval extra's google-cloud-aiplatform bound has no upper cap, unlike all/gcp/test #7009

Problem:
The eval optional dependency allowed google-cloud-aiplatform 2.x, while the all, gcp, and test extras already constrain the same dependency below 2. Users installing google-adk[eval] could therefore resolve a different major line from the sibling extras.

Solution:
Add the <2 upper bound to google-cloud-aiplatform[evaluation] in the eval extra. Add a release-dependency regression test that verifies the lower bound remains admitted and the 2.x line is excluded.

Testing Plan

Unit Tests:

• python -m pytest tests/unittests/test_release_dependencies.py -q -k eval_extra_caps_google_cloud_aiplatform_at_v2 (1 passed)
• python -m compileall -q tests/unittests/test_release_dependencies.py
• git diff --check

The remaining tests in tests/unittests/test_release_dependencies.py require the repository's installed ADK dependencies; the focused dependency-contract test passes in this checkout.

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