Upgrade Fern version from 3.48.2 to 4.15.5

[jon-fern]

Summary

Bumps the pinned Fern CLI version in fern.config.json from 3.48.2 to 4.15.5. This is the docs-starter template repo used in the FTUX onboarding flow, so new users will start with the latest Fern CLI version.

Review & Testing Checklist for Human

• Verify v4 compatibility: This is a major version bump (3.x → 4.x). Confirm that the existing template files (openapi.yaml, asyncapi.yaml, docs.yml, generators.yml) are fully compatible with Fern CLI v4 — run fern check and fern generate --docs --preview locally against this repo.
• Check CI workflows: Verify that fern-api/setup-fern-cli@v1 in the GitHub Actions workflows respects the fern.config.json version pin, or if it installs a different version independently.
• Confirm 4.15.5 is the desired target version (latest stable as of this PR).

Notes

• A companion PR is being created in fern-api/fern-platform to update the inline copy of this template used by the dashboard FTUX flow.
• Requested by: @jon-fern
• Devin Session

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