Fix virtual Postgres source_file URI backslash normalization on Windows (#1672)

[raman118]

Summary

This PR resolves issue #1672, where introspect_postgres() produces invalid source_file URIs on Windows (e.g., postgresql:\myhost\mydb instead of postgresql:/myhost/mydb).

Cause

In graphify/pg_introspect.py, the virtual_path was constructed using pathlib.Path(f"postgresql://{host}/{dbname}"). On Windows, this instantiates a WindowsPath object, which normalizes the path separators to backslashes (\). Because the path is a synthetic URI rather than a local file path, it should be OS-independent and retain forward slashes.

Solution

• Imported and utilized pathlib.PurePosixPath to construct virtual_path. Since PurePosixPath uses POSIX path semantics regardless of the host operating system, the path string uses forward slashes on all platforms.
• Added the regression test test_pg_introspect_uri_forward_slashes in tests/test_pg_introspect.py to assert that the generated virtual paths always contain forward slashes and no backslashes, even when run on Windows.
• Successfully verified that all postgresql introspection tests pass on Windows.
• Executed graphify update . to rebuild the AST knowledge graph.