fix: psycopg2 Import Handling in PostgresIndex

[m0n0x41d]

Summary of Changes

1. Fixed the import of psycopg2 by moving it from the PostgresIndex class definition to the module level with proper TYPE_CHECKING guards
2. Removed redundant import attempt inside the class definition that was failing because imports in class bodies execute at class definition time, not instance creation
3. Updated CONTRIBUTION.md to recommend uv sync --extra all instead of uv pip install -e .[dev] for development environment setup, ensuring all dependencies (including postgres) are properly installed

Notes

• I did not bump the package version as I am not a maintainer of this repository (version updates should be handled by the maintainers)
• I did not add tests for this change since the repository currently does not have tests for the indexes functionality

[jamescalam]

@m0n0x41d I don't think we can move the import from the class definition to the top-level file as it means we will see this error https://github.com/aurelio-labs/semantic-router/actions/runs/14444373527/job/40501518537?pr=576#step:6:32 will occur whenever someone tries to use the lib without postgres installed

Other changes to contributing guidelines are good

[m0n0x41d]

@jamescalam Hello, thank you for taking a look here.

I am incredibly sorry for being blind to it; you are right that we can't break an import of a base library when the optional dependency is not installed.

However, as I wrote in the related issue, the import in the instance constructor is simply not working because of Python's namespace nature.

I've pushed a small fix, moving an exception raise into init, based on a simple flag we are setting in the try/except import. Please take a look and run tests when you have spare time.