docs: improve Python custom code page with model extension guidance and bug fixes

[devin-ai-integration]

Summary

Updates the Python "Adding custom code" page based on user feedback that the page only covered extending the base client class, but didn't explain how to add custom methods to returned objects (models/types).

New content:

• Added a new "Adding custom methods to models" section showing how to extend Fern-generated Pydantic models with custom methods, computed properties, or domain logic. Covers the full workflow: creating a custom model file, .fernignore, exporting via additional_init_exports, and consuming.

Bug fixes in existing code examples:

• Fixed Python comments using // (JavaScript syntax) instead of #
• Removed an accidentally duplicated/nested my_helper method definition

Cleanup:

• Normalized indentation throughout the page (several sections had inconsistent indentation levels)
• Renamed "Adding custom SDK methods" → "Adding custom client methods" to distinguish it from the new model section

Review & Testing Checklist for Human

• Verify the model extension pattern is accurate: Confirm that extending a Fern-generated Pydantic model (e.g., from .types.plant import Plant as BasePlant) and using additional_init_exports to re-export it is the recommended approach. Check that this doesn't conflict with the existing generated Plant export in __init__.py.
• Check rendered page in preview: Verify all <Steps>, <Note>, and code blocks render correctly — the indentation cleanup was extensive and could affect MDX parsing.
• Validate the additional_init_exports config snippet: Ensure the YAML example matches the actual generator config schema (the from and imports fields).

Notes

• Triggered by on-page feedback: "how do we add custom methods to returned objects, not just the base client class?" on the /learn/sdks/generators/python/custom-code page.
• The new model extension example uses plant-themed content consistent with docs conventions.

Link to Devin session: https://app.devin.ai/sessions/9f4bdfc7973a4002a03df64bc59facc9

[devin-ai-integration]

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[fern-api]

Requested by: devin
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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1775755096-python-custom-code-improvements.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/generators/python/custom-code