Document casing sub-field for x-fern-enum extension

[Swimburger]

Summary

Adds a new "Custom casing" section to the x-fern-enum extension docs page, documenting the casing sub-field that allows per-language casing overrides for enum values.

Changes:

• Updated frontmatter (title, headline, description) to mention casing
• Added ## Custom casing section after the existing "Custom names" section
• Documents the four optional sub-fields: snake, camel, screamingSnake, pascal
• Includes a YAML example with line highlights
• Explains how casing interacts with name

Review & Testing Checklist for Human

• Verify casing sub-field names and semantics match the actual Fern implementation — the four fields (snake, camel, screamingSnake, pascal) and their described language mappings (Python, TS/Java, Go, C#) were taken from the task description and not verified against source code
• Check the YAML example is valid and the line highlights {13-17} point to the correct lines (the casing block)
• Preview the page to confirm the updated title renders well in navigation and the new section looks good

Notes

• Vale style checks were not run locally (Vale CLI not installed on the build machine); CI should catch any violations.
• No new pages or links were added, so no navigation or redirect changes needed.

Link to Devin session: https://app.devin.ai/sessions/2b9bd7d4bb33434997ba0f902132d39d
Requested by: @Swimburger

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774119568-document-casing-field.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/openapi/extensions/enum-descriptions-and-names

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.