docs: document env and client-default on global headers

[devin-ai-integration]

Summary

A global header in a Fern API config (generators.yml or a Fern Definition api.yml) can now declare a runtime default value and an environment variable that supplies it. This means SDK users get sensible header defaults — an API version header, for example — without passing them on every call, and can override the value per environment with an env var. Every SDK language resolves the value the same way, so behavior no longer differs between TypeScript, Java, and the rest. The docs previously listed only name and type for generators.yml headers and never mentioned env or client-default, so this capability was invisible.

Changed pages:

• fern/products/sdks/reference/generators-yml-reference.mdx (most changed) — the api.headers param now lists all supported keys (name, type, env, client-default, default), clarifies default as documentation-only versus client-default as the runtime SDK default, states the resolution order (explicit value > env > client-default) and that it's identical across TypeScript, Java, Python, Go, C#, Ruby, and PHP. The advanced example was also corrected to valid api.specs / api.headers structure.
• fern/products/api-def/openapi/extensions/global-headers.mdx — the generators.yml section previously showed an invalid api: list form and omitted env/client-default; both fixed, with a pointer to the reference.
• fern/products/api-def/ferndef/api-yml/global-headers (ferndef/api-yml/global-configuration.mdx) — adds the object form of an api.yml global header with env and client-default, plus the resolution order.
• fern/products/sdks/deep-dives/configure-global-headers.mdx — the SDK-facing deep dive now shows the env + client-default declaration and explains the precedence a caller sees.

Validation: pre-commit run --files <changed> (only pre-existing Vale suggestions remain) and fern check --warnings → 0 errors.

Link to Devin session: https://app.devin.ai/sessions/106856f134c9489c8b5b78689ef353b6

[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-1784999233-global-header-client-default.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/ferndef/api-yml/global-headers
• learn/api-definitions/openapi/extensions/global-headers
• learn/sdks/deep-dives/global-headers
• learn/sdks/reference/generators-yml