fix(weaviate): delete vectors by document ID

[yuya2017]

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Summary

Fixes #39175

GitHub did not allow the external issue author to self-assign #39175, so maintainer assignment is still needed.

This PR fixes an identity mismatch in the Weaviate adapter:

• New objects now inherit BaseVector._get_uuids() and use the segment doc_id instead of a UUID derived only from page_content.
• delete_by_ids() deletes by the stored doc_id property instead of treating logical segment IDs as Weaviate object UUIDs.
• Property-based deletion also removes legacy objects whose UUIDs were derived from content.
• Regression coverage verifies deletion and prevents identical text from overwriting segments with different IDs.

This also addresses the storage collision described in #36137. Unlike the existing proposal in #36140, it aligns Weaviate with the shared BaseVector identity contract and fixes deletion of both new and legacy objects.

Screenshots

Not applicable; this is a backend vector-store fix.

Validation

• make test TARGET_TESTS=./api/providers/vdb/vdb-weaviate/tests/unit_tests/test_weaviate_vector.py — 40 passed.
• Targeted ruff format --check and ruff check for the three changed files — passed.
• The Weaviate integration test was attempted against an existing local stack, but that stack does not expose gRPC port 50051; the run timed out during batch insertion. The temporary test collection was removed. The regression assertion is included for CI's Docker-backed Weaviate test environment.
• Targeted Pyrefly reported four existing errors in unchanged lines of weaviate_vector.py; no error points to this diff.

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods