Fixes #28617

Problem

The env helper rejected interface-based environment variable types because the previous generic constraint
Env extends Record<string, string | undefined> required an index signature that interfaces don’t have.
This caused TS2344 when following the documented example.

Fix

The type definition of env was updated and a new EnvKey helper type was introduced to filter valid keys.
This allows both interfaces and type aliases to work as expected without changing runtime behavior.

Changes Made

• Updated env signature using a simplified generic pattern
• Added EnvKey to restrict keys to string | undefined values
• Added a type-level test ensuring interface + type alias compatibility
• Kept all runtime logic untouched

Result

• Interface support now matches documentation
• Type aliases continue to work
• Compile-time validation preserved
• Backwards-compatible with no runtime impact