feat(vectordb): add openGauss DataVec adapter with standalone and dis…

[LinQiang391]

…tributed support

Add an opengauss VectorDB backend following the adapter integration guide:

• map eight logical ANN index types (hnsw/ivfflat/diskann plus PQ and RabitQ variants) to DataVec access methods with validated build_params and search_params, applied via transaction-local GUCs
• verify physical indexes through pg_indexes before persisting metadata; training-type indexes materialize once data exists (bulk-ingest aware) and never silently degrade to sequential scans
• support spq distributed clusters: hash-distributed collection and metadata tables, startup environment validation, and SET LOCAL spq.propagate_set_commands='local' so search parameters reach DN shard scans (spq defaults to none)
• pool connections with per-thread checkout and rollback-safe error paths
• ship an opengauss extra (psycopg2-binary), bilingual guides, and unit plus DSN-gated integration tests

Description

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

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Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
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  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

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