fix(generator-cli): consume FAI NDJSON heartbeat stream in AutoVersionStep

[devin-ai-integration]

Description

Linear ticket: Refs (none)

Client half of fern-api/fern-platform#14449. AutoVersionStep.analyzeViaFaiService now sends Accept: application/x-ndjson to POST /sdks/analyze-commit-diff so FAI can stream heartbeat lines while a multi-minute analysis of a large SDK diff runs, instead of sitting silent past the ALB idle timeout (the 504s that motivated fern-api/fern-platform#14376).

Behavior is negotiated on the response Content-Type, so a FAI deployment that has not picked up the server change (or any non-streaming proxy) still works via the existing response.json() path.

Changes Made

• AutoVersionStep.ts: add Accept header; when the response is application/x-ndjson, readFaiAnalyzeStream reads response.body line by line:

  {"type":"heartbeat"}                 -> ignored
  {"type":"result","result":{...}}     -> returned, then validated by isFaiAnalyzeResponse as before
  {"type":"error","status":413,...}    -> throw (HTTP status is already 200 by then)
  stream ends w/o result               -> throw
  

  Throwing keeps the existing PATCH fallback in the caller.
• Tests: ndjsonResponse helper that splits the stream across arbitrary chunk boundaries; new cases for heartbeats-then-result, in-band error, and truncated stream; assert the Accept header on the existing plain-JSON test.
• changes/unreleased/autoversion-fai-ndjson-stream.yml (fix).
• Updated README.md generator (not applicable)

Testing

• Unit tests added/updated — auto-version-step.execute.test.ts: 44 passed
• pnpm turbo run compile --filter @fern-api/generator-cli clean; biome check clean
• Manual testing against a deployed FAI with the server PR — pending that deploy

Rollout: FAI PR deploys first (harmless either way, since the header is opt-in on both sides); generators pick this up on their next version bump/Docker rebuild once the catalog pin is updated.

Link to Devin session: https://app.devin.ai/sessions/7e579645945144bd8566e4a37d79ae66
Open in Devin Desktop: https://app.devin.ai/desktop/session/7e579645945144bd8566e4a37d79ae66?variant=devin

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AI Review Summary

Adds NDJSON streaming support to AutoVersionStep's FAI call with a content-type negotiated fallback to plain JSON. Logic is mostly sound; a couple of robustness nits around CRLF/\r handling and unknown event types being fatal.

• 🟡 1 warning(s)
• 🔵 2 suggestion(s)

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[devin-ai-integration]

Devin Review found 1 potential issue.