fix(cli): make AsyncAPI parsing failures fatal

[tjb9dc]

Description

Refs: Slack thread about AsyncAPI parsing failures being silently skipped

Previously, when an AsyncAPI document failed to parse, it was logged at debug level with "Skipping parsing document" and the CLI continued. This made it easy to miss configuration issues. Now, AsyncAPI parsing failures will cause fern check and generation to fail with an error.

Link to Devin run: https://app.devin.ai/sessions/69b905a8948a427485624f081ccc25f4
Requested by: @tjb9dc

Changes Made

• Removed try-catch around AsyncAPI parsing in parse.ts - errors now propagate up
• Removed try-catch around AsyncAPI loading in OpenAPILoader.ts - loading failures now propagate up
• OpenAPI parsing behavior unchanged (still logs and skips on failure)
• Added version 3.64.3 changelog entry

Testing

• Unit tests added/updated
• Manual testing completed (lint checks pass)

Human Review Checklist

• Verify this is a breaking change for users with invalid AsyncAPI documents that were previously silently skipped
• Confirm the asymmetric behavior is intentional: AsyncAPI failures are now fatal, while OpenAPI failures still skip silently
• Consider if users need a way to opt-out of this stricter behavior

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