feat: add alpha, preview, and stable as availability options for x-fern-availability

[devin-ai-integration]

Description

Refs: User request to add alpha, preview, and stable as availability options per industry standards.

Adds alpha, preview, and stable as new values for the x-fern-availability OpenAPI extension. These map to existing internal availability statuses:

• alpha → IN_DEVELOPMENT
• preview → PRE_RELEASE (same as beta)
• stable → GENERAL_AVAILABILITY (same as ga)

Link to Devin run: https://app.devin.ai/sessions/5732bfb11d0849959b0fcb3cc3abe788
Requested by: Niels Swimberghe (@Swimburger)

Changes Made

• Updated x-fern-availability.ts in v3-importer-commons to handle alpha, preview, and stable values
• Updated getFernAvailability.ts in openapi-ir-parser to handle alpha, preview, stable, and in-development values
• Added InDevelopment to the openapi-ir Availability enum and regenerated the SDK
• Added test endpoints (/alpha-endpoint, /preview-endpoint, /stable-endpoint) to availability test fixtures
• Updated all related test snapshots

Testing

• Unit tests updated (snapshots regenerated)
• Lint checks pass (pnpm run check)

Review Checklist

• Verify the mapping of alpha → IN_DEVELOPMENT, preview → PRE_RELEASE, and stable → GENERAL_AVAILABILITY aligns with expectations
• Note: In the older openapi-ir-to-fern conversion path (openapi/availability.json snapshot), the anAlphaEndpoint shows "availability": undefined rather than "in-development". This appears to be a pre-existing limitation where that converter doesn't handle InDevelopment. The v3 importer path correctly shows "status": "IN_DEVELOPMENT".

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