fix(cli): gracefully handle docs resolution failures in missing-redirects rule

[paarthfern]

Description

Follow-up to #15853 which added the core try-catch fix for the missing-redirects rule. This PR adds the assertNever exhaustiveness guard to the makeSkipVisitor switch statement per CLAUDE.md conventions, and includes a changelog entry.

Changes Made

• Added default: assertNever(fetchResult) to the makeSkipVisitor switch on the FetchResult discriminated union, ensuring compile-time safety when new variants are added
• Added assertNever import from @fern-api/core-utils
• Added unreleased changelog entry for the fix

Testing

• All 145 unit tests in @fern-api/docs-validator pass
• pnpm lint:biome passes
• pnpm format passes
• TypeScript compilation succeeds

Link to Devin session: https://app.devin.ai/sessions/9e60e54a91c84401a1d139ae7e2fb655
Requested by: @paarthfern

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[devin-ai-integration]

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 2 additional findings in Devin Review.

[devin-ai-integration]

🟡 Switch on discriminated union FetchResult lacks default: assertNever() for exhaustiveness

The makeSkipVisitor function switches on fetchResult.type (a discriminated union) but does not include a default case calling assertNever. Per CLAUDE.md: "Every switch statement on a discriminated union's type field must include a default case [that] call[s] assertNever from @fern-api/core-utils." The same package already follows this pattern in validateDocsWorkspace.ts:31-32. While TypeScript's return-type checking provides partial safety here, a new variant added to FetchResult without updating makeSkipVisitor would not produce a compile error in the default branch — only a potentially confusing implicit-undefined-return error.

Suggested change

	case "resolve-failed":
	return {
	file: () => [
	{
	severity: "warning",
	message:
	`Missing redirects check skipped: failed to resolve docs definition (${fetchResult.reason}). ` +
	"This can happen after a major CLI upgrade that changes type naming. " +
	"Publish your docs once to update the baseline, then re-run this check."
	}
	]
	};
	case "resolve-failed":
	return {
	file: () => [
	{
	severity: "warning",
	message:
	`Missing redirects check skipped: failed to resolve docs definition (${fetchResult.reason}). ` +
	"This can happen after a major CLI upgrade that changes type naming. " +
	"Publish your docs once to update the baseline, then re-run this check."
	}
	]
	};
	default:
	assertNever(fetchResult);

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[devin-ai-integration]

Good catch — added default: assertNever(fetchResult) and the assertNever import in 789f56e.