Add PostgreSQL schema support to Memory and SQLAlchemyChatStore

[vrushab-bit]

Description

This PR adds PostgreSQL schema support to the Memory module and SQLAlchemyChatStore, allowing users to organize chat data into specific database schemas for better data organization and multi-tenancy scenarios.

Changes Made:

• Added db_schema parameter to Memory.from_defaults() method
• Enhanced SQLAlchemyChatStore to support schema specification via db_schema parameter
• Implemented automatic schema creation for PostgreSQL and other schema-supporting databases
• Added proper handling for SQLite (which doesn't support schemas) to maintain compatibility
• Added comprehensive unit tests for both Memory and SQLAlchemyChatStore classes

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 - This is an enhancement to existing core functionality

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 - This is an update to llama-index-core package

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?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

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

Test Coverage:

• 9 comprehensive unit tests covering schema functionality
• Tests for PostgreSQL schema creation and SQLite compatibility
• Integration tests for Memory class operations with schema
• Serialization/deserialization tests
• Parameter validation and edge case handling

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

[vrushab-bit]

Request to Rerun the failed job

[logan-markewich]

Seems straightforward. Thanks for adding tests