fix: support x-displayName extension on OpenAPI tags

[jon-fern]

Description

Refs Slack thread — Phil Freo reported that:

1. Tag names like "WhatsApp" get mangled to "Whats App" because of titleCase processing
2. The x-displayName extension on tags is not being respected in the documentation sidebar

This PR adds support for the x-displayName OpenAPI extension on tags, allowing users to explicitly control the display name used in Fern definitions and docs navigation.

Changes Made

• Added displayName optional field to the OpenAPI IR Tag type and regenerated SDK types
• Updated generateIr.ts to extract x-displayName from OpenAPI tag objects
• Updated buildServices.ts fallback chain: group?.summary → irTag?.displayName → irTag?.id
• Updated DocsDefinitionResolver.ts to pass displayName through openApiTags
• Updated ApiReferenceNodeConverter.ts to use displayName for tag description page headings
• Updated existing tests to include the new displayName field
• Added tag-display-names test fixture with x-displayName examples
• Added CLI changelog entry (v4.46.3)

Important Review Notes

• Partial fix: This gives users a workaround via x-displayName, but tags without x-displayName will still go through titleCase, which can mangle names like "WhatsApp" → "Whats App". Addressing the root titleCase behavior is a larger change that may warrant a separate PR.
• Extension access pattern: Uses (tag as unknown as Record<string, unknown>)["x-displayName"] since the OpenAPI TS types don't expose extension properties. This is a common pattern in the codebase.
• No IR migration needed: The displayName field is on the OpenAPI IR (openapi-ir), not the main Fern IR, so no IR migration is required.

Testing

• Existing tests updated to match new type signatures
• Local compilation verified (pnpm compile passes for all affected packages)
• Biome lint check passes (pnpm run check)
• Seed test output not generated for the new fixture (fixture has no generator entries)

Link to Devin session: https://app.devin.ai/sessions/83ca6010faff45f89f5ac888d9389227
Requested by: @jon-fern

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[devin-ai-integration]

Closing this PR to recreate with correct title format (missing scope). New PR incoming.