Add GIN index support for text array metadata in PostgreSQL vector store

[Jash271]

Description

Enable automatic GIN index creation for text array metadata fields in the PostgreSQL vector store. This allows fast array membership queries using PostgreSQL operators (?|, ?&, @>) without requiring raw SQL workarounds.

Changes:

• Extended PGType to support text[] array type
• Modified get_data_model() to create GIN indices for array fields and BTREE indices for scalar fields
• Cast metadata to JSONB in GIN index creation for compatibility with both JSON and JSONB columns
• Added comprehensive tests covering GIN index creation, CONTAINS, ANY, ALL operators, and mixed BTREE/GIN queries
• Tests use PostgreSQL system catalogs (pg_class, pg_am) for type-safe index verification

This maintains backward compatibility - existing BTREE indices continue working, and GIN indices are only created when text[] is specified in indexed_metadata_keys.

Fixes #20128

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

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

How Has This Been Tested?

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• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• 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 ran uv run make format; uv run make lint to appease the lint gods

[AstraBert]

Lgtm :)