[Feature Request] Add API balance endpoint to retrieve account balance information

Bug Description
Currently, Anthropic does not expose a balance endpoint for users API balance. It would be useful to allow this, so that apps could warn the user when their balance is low, encourage refill, and explain why their request failed.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.47
• Feedback ID: edd3eec6-0c75-42a3-837f-111178ca2243

Errors

[{"error":"Error: NON-FATAL: Lock acquisition failed for /Users/mjy/.local/share/claude/versions/2.1.47 (expected in multi-process scenarios)\n    at hqA (/$bunfs/root/claude:3483:2098)\n    at XhL (/$bunfs/root/claude:3483:1209)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-19T16:02:55.934Z"}]