fix: validate refresh token exchange response at runtime

[marcusolsson]

Issue 12 from TODO.md

The refreshAccessTokenInternal function used an unchecked TypeScript as cast after parsing the refresh response. If the Berget /v1/auth/refresh endpoint (or a proxy) returns a 200 OK with a malformed body, the plugin stores undefined as the new access token and overwrites the previously valid token in OpenCode's persistent storage.

Changes

• Replaces the as cast with explicit typeof runtime validation for token and expires_in.
• Returns {success: false, reason: 'Invalid token response from refresh endpoint'} on malformed bodies.
• Uses typeof data.refresh_token === 'string' to safely extract rotated refresh tokens.
• Adds 3 regression tests for missing token, wrong expires_in type, and proxy-wrapped error body.

Refs: TODO.md Issue 3 (same pattern) / Issue 12