test: improve vector store tests

[bowenliang123]

Description

• fix: avoid mixed usage of the concepts between doc_id and dataset_id in tests
• fix the method signature of delete_by_ids in milvus and qdrant, by renaming doc_ids to ids as in the overridden method of the parent class
• fix the typo of sample to example
• simplify code in vector store tests
• add assertion to text_exists test
• make sure delete_by_ids deleting the newly inserted docs
• remove unused import in vdb tests
• update CI job to squash vdb startup steps into one for less time cost (shrink ~30s to ~13s) and move to where after running other unit tests

Type of Change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update, included: Dify Document
• [x] Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• [ ] Dependency upgrade

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [ ] TODO

Suggested Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods
• [ ] optional I have made corresponding changes to the documentation
• [ ] optional I have added tests that prove my fix is effective or that my feature works
• [ ] optional New and existing unit tests pass locally with my changes

[bowenliang123]

cc @takatost @JohnJyong