fix(cli): use OVERRIDE_FDR_ORIGIN for air-gap detection in self-hosted environments

[thesandlord]

Description

Refs: Slack thread from @thesandlord about fern generate --docs stalling in air-gapped environments

Link to Devin run: https://app.devin.ai/sessions/dcec6a094378444b94e7253cc9c06cf4
Requested by: Sandeep Dinesh (@thesandlord)

In self-hosted environments, the air-gap detection was checking connectivity to https://registry.buildwithfern.com/health (from DEFAULT_FDR_ORIGIN) instead of the local FDR server at http://localhost:8080/health (from OVERRIDE_FDR_ORIGIN). This caused the CLI to incorrectly detect air-gapped mode even when the local FDR server was reachable.

The FDR client already respects OVERRIDE_FDR_ORIGIN, but the air-gap detection did not.

Changes Made

• Updated publishDocs.ts and runRemoteGenerationForGenerator.ts to use OVERRIDE_FDR_ORIGIN ?? DEFAULT_FDR_ORIGIN for air-gap detection, matching the FDR client behavior
• Improved debug logging for all air-gap checks to show:
  • The URL being tested
  • The timeout value
  • The result (connected/air-gapped) with error details when applicable
• Added changelog entry for version 3.51.3

Testing

• Manual testing in air-gapped environment (requires self-hosted deployment)
• Lint checks pass

Human Review Checklist

• Verify the environment variable precedence (OVERRIDE_FDR_ORIGIN ?? DEFAULT_FDR_ORIGIN ?? fallback) is correct
• Confirm no other files need this fix (searched for DEFAULT_FDR_ORIGIN usage in air-gap detection)

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