feat(cli): add x-fern-playground-description for playground-only auth docs

[adidavid014]

Description

Lets a customer author auth guidance that only shows in the API Playground (under the token input), without touching the API reference's Authorization description.

components:
  securitySchemes:
    dropboxTeamAuth:
      description: Team-level OAuth 2.0 token…      # unchanged → API reference
      x-fern-playground-description: |             # new → playground only
        To mint a token, see [Testing with a generated token](https://docs.dropboxapi.com/…)

Fern definition equivalent: auth-schemes.<name>.playground-docs.

Flow: x-fern-playground-description / playground-docs → IR AuthScheme.playgroundDocs (optional) → FDR playgroundDescription on bearerAuth / basicAuth / header. docs/description are untouched, so the reference page and the collapsed playground auth header keep their existing text.

Rendering side is fern-api/fern-platform#14486 (adds playgroundDescription to the FDR zod schemas and renders it as a callout under the input). This PR's @fern-api/fern-platform-published fdr-sdk types don't have playgroundDescription yet, so @fern-api/register#compile fails until that PR merges and the SDK is bumped — hence draft.

Changes Made

• IR: BaseAuthScheme.playgroundDocs?: string, VERSION 67.23.0 → 67.24.0, changelog entry, regenerated SDK. Additive/optional → no migration.
• OpenAPI 3.1 converter reads the extension for bearer/basic/header/oauth; OAuth endpoint resolution preserves it.
• Fern definition schema: new WithPlaygroundDocsSchema mixed into header/basic/bearer/inferred/oauth schemes; both convertApiAuth converters pass it through.
• ir-to-fdr-converter: maps playgroundDocs → playgroundDescription (bearer/basic/header + inferred and OAuth bearer fallbacks).
• CLI changelog entry under changes/unreleased/.
• Not changed: the legacy openapi-ir-parser v3 convertSecurityScheme path, which already drops the security scheme description entirely.

Testing

• Unit tests added/updated — new fixture auth-playground-description in register/src/ir-to-fdr-converter/__test__ asserting IR playgroundDocs and FDR playgroundDescription coexist with description; auth snapshots updated.
• Manual testing completed — end-to-end browser verification of the rendered callout happens on the fern-platform PR.

Link to Devin session: https://app.devin.ai/sessions/6a812925d9ba495aa86d3cc6a6f3f7b0
Open in Devin Desktop: https://app.devin.ai/desktop/session/6a812925d9ba495aa86d3cc6a6f3f7b0?variant=devin
Requested by: @adidavid014

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