fix: support Python 3.9 type generation

[sornapudisuresh]

Summary

• import NotRequired and TypeAlias from typing_extensions
• support generated Python types on Python 3.9 and 3.10
• add a focused regression test and update the integration snapshot

Test plan

• npm run check
• Prettier check passed
• focused Python generator unit test passed
• Python generator integration test passed
• imports validated with Pydantic on Python 3.9, 3.10, and 3.11

Fixes #1088

[tushardev-365]

Ran the 3.9/3.10 smoke test I offered. Took your branch's generated header (the typing_extensions import) with a TypedDict using NotRequired plus a TypeAlias, and it imports cleanly on both 3.9 and 3.10. The pre-fix form (those two names from typing) still throws the original ImportError on both, and 3.12 passes either way as expected. Fix looks good.

[sornapudisuresh]

Thanks @tushardev-365 for verifying both the pre-fix failure and corrected imports on Python 3.9 and 3.10!