[Fix] Fix waiting for `databricks_vector_search_index` readiness

[alexott]

Changes

Change the wait condition from waiting for the index to be fully ready to either fully ready or started indexing.

Resolves #4144

Tests

• [x] make test run locally
• [ ] relevant change in docs/ folder
• [ ] covered with integration tests in internal/acceptance
• [x] relevant acceptance tests are passing
• [ ] using Go SDK

[github-actions[bot]]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger: go/deco-tests-run/terraform

Inputs:

• PR number: 4243
• Commit SHA: 090a48881dd01e7ce801e09dd1ef2d2317ffaf5f

Checks will be approved automatically on success.

[eng-dev-ecosystem-bot]

Test Details: go/deco-tests/11907832306