fix(cli): make docs-validator rule init failures honor configured severity

[fern-support]

Description

Linear ticket: Refs

When a docs-validator rule throws an exception during its asynchronous create() step, the validator emits a Rule "X" failed to initialize violation. Previously this violation was hardcoded to severity: "fatal", which meant configuring a rule at warn in docs.yml had no effect on init-time failures — fern check would still exit 1.

Reported by a customer (NVIDIA-NeMo/Gym) whose CI was failing on:

[docs] 1 error
    [error]
        path: docs.yml
        issue: Rule "missing-redirects" failed to initialize: [object Object]

…despite having check.rules.missing-redirects: warn configured in docs.yml. The rule itself already had defensive makeSkipVisitor paths that emit warning severity for things like FDR fetch failures, but an exception thrown from DocsDefinitionResolver.resolve() bypassed those and bubbled out of create(), hitting the hardcoded-fatal branch in validateDocsWorkspace.ts.

The [object Object] portion was a separate bug: String(error) was being called on a thrown non-Error value with no useful toString.

Changes Made

• validateDocsWorkspace.ts: when rule.create() throws, look up the rule's configured severity (severityOverrides) and use it for the resulting violation. warn → warning, error → error, no override → fatal (preserves existing behavior for any rule without an explicit override).
• formatInitError.ts (new): format unknown thrown values into a readable message. Plain objects are serialized to JSON; circular / empty objects fall through to Object.prototype.toString.call(...). Eliminates [object Object] in user-facing messages.
• rules/missing-redirects/missing-redirects.ts: wrap DocsDefinitionResolver.resolve() + the v1→latest navigation migration in a try/catch that returns a new makeSkipVisitor({ type: "resolve-failed" }) (rendered as a warning). This is a defense-in-depth fix: even on a CLI version without the framework-level severity change above, the missing-redirects rule no longer crashes check when local navigation fails to resolve.
• __test__/formatInitError.test.ts (new): unit tests covering Error, string, plain object, empty object, circular object, null, and undefined inputs.
• Changelog entry under packages/cli/cli/changes/unreleased/.
• Updated README.md generator (if applicable)

Testing

• Unit tests added/updated (pnpm test --filter @fern-api/docs-validator → 151/151 passing, 6 new)
• Manual testing completed — built the dev CLI and ran fern check against the customer's repo (NVIDIA-NeMo/Gym PR [openapi linter] require x-server-name for servers #1299, commit bad8dead2); rule degrades gracefully without modifying their docs.yml.
• pnpm lint:biome --fix and pnpm format:fix clean.

Link to Devin session: https://app.devin.ai/sessions/bd3a58d7c2c44de0b1f23ea520709888
Requested by: @fern-support

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review to trigger a review and subscribe this PR to future pushes, or @claude review once for a one-time review.

_{Tip: disable this comment in your organization's Code Review settings.}

[fern-support]

@claude review once

[claude]

⚠️ Code review skipped — your organization's overage spend limit has been reached.

Code review is billed via overage credits. To resume reviews, an organization admin can raise the monthly limit at claude.ai/admin-settings/claude-code.

Once credits are available, comment @claude review on this pull request to trigger a review.