feat: add create_extension flag to init_vectorstore_table

[Michele Catalano (ironpinguin)]

Summary

Add a create_extension parameter (default True) to _ainit_vectorstore_table,
ainit_vectorstore_table, and init_vectorstore_table. When set to False, the
CREATE EXTENSION IF NOT EXISTS vector statement is skipped.

Motivation

Some deployments use managed databases (e.g. AlloyDB, Cloud SQL) where the pgvector
extension is pre-installed, or the database user lacks CREATE EXTENSION privileges.
Forcing the extension creation on every table init causes unnecessary permission errors.

Changes

• langchain_postgres/v2/engine.py: added create_extension: bool = True parameter
  to the three init_vectorstore_table variants; wrapped the CREATE EXTENSION call
  in a conditional block.
• tests/unit_tests/v2/test_engine.py: added test_init_table_no_create_extension
  for both async and sync paths.

Usage Example

# Skip extension creation (extension already installed or insufficient privileges)
await engine.ainit_vectorstore_table(
    table_name="my_table",
    vector_size=768,
    create_extension=False,
)

Breaking Changes

None. The new parameter defaults to True, preserving existing behavior.