docs: document TypeScript passthrough fetch auth origin scoping

[devin-ai-integration]

Summary

Generated TypeScript SDKs now send their auth credentials on passthrough client.fetch() calls only when the request targets the SDK's own base URL origin. This matters for customers because passing an absolute third-party URL into the escape hatch previously forwarded the end user's API token to that host; SDK consumers no longer have to sanitize URLs themselves. The behavior shipped in fern-api/fern#17208, and the escape hatch had no prose coverage in the public docs (only SDK changelog entries), so this adds a short section documenting both the method and the origin rule.

Pages changed

• fern/products/sdks/deep-dives/generated-sdk.mdx (SDK user features) — new ## Unmodeled endpoints section: what client.fetch() is, that it reuses the client's base URL/headers/timeouts/retries/logging, and the auth rule (relative and same-origin absolute URLs get credentials; cross-origin absolute URLs, including a different port, don't and must supply their own credential headers; no base URL configured means no auth headers; non-auth headers always sent).

Link to Devin session: https://app.devin.ai/sessions/872b1bd7bcb441db8b40472c2f03bbca

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🌿 Preview your docs: https://fern-preview-devin-1784999160-ts-passthrough-auth-origin.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/deep-dives/sdk-user-features