fix(cli): fix false "page was moved" warnings for versioned docs pages

[devin-ai-integration]

Description

Fixes false "page was moved" warnings in fern check and fern docs dev when the same .mdx file is intentionally referenced across multiple doc versions.

Changes Made

• buildPageIdToSlugMap: Changed return type from Map<string, string> to Map<string, Set<string>> so that all slugs are tracked per pageId (instead of the last one overwriting earlier entries)
• findRemovedSlugs: Updated to accept the new multi-slug map type. The activeSlugs set now correctly contains all slugs from all navigation entries, preventing false positives when the same file appears under multiple version slugs
• Tests: Updated all existing tests for the new type and added two new test cases covering the versioned-page scenario (single and multi-version)
• Changelog: Added unreleased changelog entry

Root Cause

buildPageIdToSlugMap returned Map<string, string>, so when the same file appeared in multiple version navigation entries (e.g., safety-notices.mdx under both v2-2-77 and v2-3-2), only the last slug survived. findRemovedSlugs then saw the published slug didn't match the single local slug and wasn't in activeSlugs, incorrectly flagging it as "moved".

Testing

• Unit tests added/updated (34 tests all passing, including 2 new versioned-page tests)
• Lint/format checks pass (biome, format)

Link to Devin session: https://app.devin.ai/sessions/9be668fc08984ce4ad8967a0bcde10d9

[devin-ai-integration]

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