Support custom column names for PgVectorStore (partial implementation)

[dylan-jung]

Description

This pull request is an initial step toward addressing
#1934.

Currently, PgVectorStore assumes fixed column names (id, content, metadata, embedding), which makes it difficult to integrate with existing database schemas. This PR introduces a column-mapping approach that allows customizing column names, starting with the most critical paths.

What’s included in this PR

1. Column name mapping for table creation and inserts

   • The table definition (CREATE TABLE) and insert/upsert logic now use configurable column names instead of hardcoded ones.
   • This establishes the core pattern for supporting custom schemas.

2. Schema validation updated accordingly (todo)

   • PgVectorSchemaValidator has been updated to validate against the configured column names rather than fixed defaults.

3. Integration tests added

   • New integration tests verify that:

     • A table is correctly created with custom column names.
     • Documents can be successfully inserted using the custom schema.
       ...(will be added after confirm)

   • Tests are implemented using the existing Testcontainers + ApplicationContextRunner style and confirm the behavior end-to-end.

What’s intentionally left out (for now)

This PR does not yet update all SQL paths (e.g. similarity search, delete, etc.).
Before expanding the change further, I wanted to confirm that this approach and API direction are acceptable to the maintainers.

If this direction looks good, I’m happy to follow up with additional PRs to:

• Apply the same mapping to the remaining queries
• Schema validation update
• Refine tests and documentation as needed

Affected files

• PgVectorStore
• PgVectorSchemaValidator
• New integration test(s) covering custom column names

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Thank you for taking the time to review this contribution.
I’m looking forward to your feedback on the approach before continuing with the remaining changes.