fix(auth): tolerate current billing/info payload and improve auth JSON errors

[powerdot]

This fixes two issues that block suno auth --login against the current Suno web API:

1. billing/info now returns a payload shape that no longer guarantees fields like plan, total_credits_left, period, etc.
   The CLI was treating those as required, so auth appeared to fail even though the Clerk exchange had already succeeded.

2. Clerk auth failures were being surfaced as a generic error decoding response body, which made it hard to distinguish a real auth problem from API schema drift.

This patch:

• makes BillingInfo, Plan, and Model more tolerant to missing fields
• falls back from total_credits_left to credits when needed
• synthesizes a minimal plan when Suno omits it
• captures raw response text before JSON parsing for Clerk and billing paths
• turns opaque decode failures into actionable API errors with a response excerpt
• clears stale Clerk session state when --jwt is used directly

Verification

Verified locally against the current live Suno web API:

• suno auth --login succeeds
• suno credits --json succeeds