fix(python): deduplicate additional_init_exports in __init__.py

[jsklan]

Description

Fixes duplicate imports in __init__.py when using additional_init_exports. When the same symbol was exported from both the regular types module and from additional_init_exports, the generator would produce duplicate imports causing F811 "Redefinition of unused" ruff errors, which caused generation to fail.

Link to Devin run: https://app.devin.ai/sessions/fa9ba4d562ab4d6ab3ebae66bef5c0de
Requested by: @jsklan (judah@buildwithfern.com)

Changes Made

• Added _deduplicate_exports method in module_manager.py that filters exports to ensure each symbol is only imported once
• When the same symbol appears in both regular exports and additional_init_exports, the additional_init_exports version takes precedence
• Added test fixture examples:additional_init_exports_with_duplicates demonstrating the fix
• Updated versions.yml with version 4.50.1 changelog entry

Testing

• Test fixture added that reproduces the duplicate export scenario
• Verified generation succeeds with the fix applied
• Lint checks pass (pnpm run check)

Human Review Checklist

• Verify the deduplication logic correctly gives precedence to additional_init_exports (relies on registration order - later entries win in the all_exports dict)
• Confirm the generated __init__.py in the fixture shows Actor, Actress, Movie imported only from .types.types (not duplicated from .types)

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