gh-156341: Fix threading local fallback import

[Sohel2309]

Fixes #156341.

When _thread._local is unavailable, importing threading falls back to the pure Python _threading_local implementation. The fallback currently creates a circular initialization path that prevents threading from importing.

This change delays _thread_local_info initialization until after the main thread is registered and updates the fallback implementation to use the threading module explicitly.

Adds a regression test for the fallback import path.

• Issue: Broken circular import between threading and _threading_local #156341

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