feat(cli): add missing-redirects rule to fern check

[kafkas]

Description

Linear ticket: FER-9171

Add a new fern check rule, missing-redirects, that compares the current docs navigation against the previously published markdown slug table stored in FDR. The rule detects when a page that was previously published has either been removed entirely or moved to a new slug without a corresponding redirect in docs.yml, helping prevent broken historical docs URLs after publish.

The rule is configurable through check.rules in docs.yml, where users can set it to warn or error, and it follows the standard docs check severity behavior. In practice this means the rule defaults to warning behavior, but can be promoted to an error when teams want redirect coverage to be enforced before publishing.

Because the rule depends on prior published state, it makes an authenticated network call to FDR during validation. The implementation handles first-publish, missing auth, offline/network failure, and timeout cases gracefully so the behavior remains consistent with the configured severity. This PR also extracts the core diffing and redirect-matching logic into unit-testable helpers and aligns slug generation behavior with the backend, including changelog-specific handling.

Changes Made

• Added a new missing-redirects docs validation rule and wired it into check.rules configuration in docs.yml
• Fetches previously published markdown slug entries from FDR and compares them against the current resolved docs navigation
• Reports pages that were removed or moved without redirect coverage, with suggested redirect entries for moved pages
• Handles first-publish, missing auth, offline/network failure, and timeout scenarios according to the configured rule severity
• Extracted pure diffing and redirect-coverage logic into testable helpers and added unit tests for removed, moved, wildcard redirect, and basepath cases
• Reused shared pageId-to-slug behavior for changelog entries to avoid drift with backend slug table generation

Testing

• Unit tests added/updated
• Manual testing completed

[claude]

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