Fixes #21050

Description

Preserve named tuple field information when matching against generic class-based NamedTuples.

This updates class-pattern narrowing so a generic named tuple type object is normalized to its named tuple instance fallback before pattern analysis. That keeps keyword field lookup on the named tuple instance type instead of the tuple alias form, so field names and concrete type arguments are preserved.

Tests

Added regressions for:

• generic NamedTuple with Generic[T] syntax in match class patterns
• PEP 695 generic NamedTuple syntax in match class patterns