docs: clarify how Fern handles OpenAPI string validation keywords

[devin-ai-integration]

Summary

Users assume maxLength and friends are enforced by generated SDKs. The OpenAPI overview page said nothing about validation keywords, so there was nowhere in the public docs to correct that.

Adds a two-sentence ## String validation keywords section to the OpenAPI overview stating that Fern reads maxLength, minLength, pattern, and format, where they surface (API Reference type display, API Explorer input cap, fern check example validation), and that generated SDKs don't enforce them — that stays the API server's job.

The equivalent statement already exists for Fern Definition on api-def/ferndef/types.mdx ("enforced by the server, but the generated client SDKs don't include validation logic"), so no new page and no changes elsewhere.

Link to Devin session: https://app.devin.ai/sessions/13be6b5189344424adada59f49cd33fc

[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, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769805-openapi-validation-keywords.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/openapi/overview