fix(generator-cli): fall back to PR description when FAI changelog entry is empty

[devin-ai-integration]

Description

Linear ticket: N/A

AutoVersionStep writes a changelog.md block for every MAJOR/MINOR bump using analysis.changelogEntry from FAI. FAI occasionally returns a bump with a populated pr_description / version_bump_reason but an empty changelog_entry, which produced a version-only changelog block with no description of the (often breaking) change.

Two fixes:

1. Whole-analysis fallback. A pure resolveChangelogEntryFallback(analysis) helper is applied at the top of both bump paths via withChangelogFallback:

if changelogEntry has text || versionBump === "PATCH" -> undefined (no change)
else first non-empty of [prDescription, versionBumpReason, commitMessageBody(message)] (trimmed)

commitMessageBody is the conventional-commit body (everything after the subject line) with a trailing 🌿 Generated with Fern trailer stripped. A warning naming the fallback source is logged when it is used. PATCH bumps are left alone since they never carry a changelog entry.

2. Chunked-analysis gap (flagged by Devin Review). analyzeChunks picks the highest bump across chunks but collects entries from every chunk, so a MAJOR chunk with an empty entry plus a MINOR chunk with text yielded a non-empty aggregate that omitted the breaking change — and fix 1 never fired. The aggregation is extracted into a pure aggregateChunkAnalyses(chunks) that tracks each entry's bump level; if no entry exists at the winning (MAJOR/MINOR) level, the winning chunk's versionBumpReason is prepended to the entries before consolidation.

Changes Made

• packages/generator-cli/src/pipeline/steps/AutoVersionStep.ts: withChangelogFallback, resolveChangelogEntryFallback(+ source-tracking variant), commitMessageBody, aggregateChunkAnalyses; analyzeChunks now delegates to it. FAIAnalysis exported for tests.
• packages/generator-cli/changes/unreleased/fix-empty-changelog-entry-fallback.yml: changelog entry (fix).
• Updated README.md generator (not applicable)

Testing

• Unit tests added: packages/generator-cli/src/__test__/autoversion-changelog-fallback.test.ts (10 cases covering both helpers) — all pass.
• pnpm turbo run compile --filter @fern-api/generator-cli, biome lint --error-on-warnings and biome format clean on touched files.
• Manual testing completed

Link to Devin session: https://app.devin.ai/sessions/403232e0a3414177b88c12a6cfbbefc2
Open in Devin Desktop: https://app.devin.ai/desktop/session/403232e0a3414177b88c12a6cfbbefc2?variant=devin

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[devin-ai-integration]

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[nitpickybot]

AI Review Summary

Adds a fallback so empty FAI changelog entries for MAJOR/MINOR bumps reuse the PR description, bump reason, or commit body. Logic is clean and well tested; main nits are a slightly misleading warning message and a possible FERN_TRAILER replace assumption.

• 🔵 2 suggestion(s)

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This review is complete for the current scope. Addressing all critical and warning findings above satisfies the review; suggestions are optional. Request another review only if subsequent changes introduce new behavior or materially expand the pull request's scope.

[devin-ai-integration]

Devin Review found 1 potential issue.