Add mocked tests for vector_databases operations (KBAAS-341)

[nbhoot157]

Add mocked tests for vector_databases operations (KBAAS-341)
Adds mocked tests for all 11 vector_databases operations, mirroring the existing test_databases.py pattern. Covers list, create (with project_id), get, update, delete, list_backups, get_restore_status, post_restore_backup, get_credentials, post_resize, and update_tags.
All tests pass (pytest tests/mocked/test_vector_databases.py -v — 11/11).
The generated client already includes the vector_databases surface from the openapi spec merge (digitalocean/openapi#1188, commit 65127f6). These tests validate the generated methods have the expected signatures and response shapes.
Ticket: KBAAS-341 (parent KBAAS-265)

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