fix: upgrade fern python SDK generator to fix mypy errors in preview-python CI

The preview-python CI check has been failing since at least Feb 2026 due to
32 mypy [no-redef] errors in auto-generated Python SDK code. The root cause
is the fern-python-sdk generator v4.37.1 producing duplicate field names in
discriminated union types (e.g., "type" defined twice).

Upgrade from v4.37.1 to v4.64.1 which includes:
- v4.41.8: Fix duplicate field names in discriminated union types
- v4.35.1: Fix SyntaxWarnings for invalid escape sequences in docstrings

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: Vapi Tasker

fern/apis/api/generators.yml

@@ -11,7 +11,7 @@ groups:
   python-sdk:
     generators:
       - name: fernapi/fern-python-sdk
-        version: 4.37.1
+        version: 4.64.1
         api:
           settings:
             prefer-undiscriminated-unions-with-literals: true