fix: use application/x-www-form-urlencoded for OAuth token requests

[coleleavitt]

Summary

Fixes "Token refresh failed: 400" errors when using Claude Pro/Max OAuth authentication.

Problem

The Anthropic OAuth token endpoint (https://console.anthropic.com/v1/oauth/token) requires application/x-www-form-urlencoded content type, but the plugin was sending application/json.

This caused 400 Bad Request errors during:

• Initial token exchange (exchange() function)
• Token refresh (in the loader fetch wrapper)

Fix

Changed both OAuth token requests to use:

• Content-Type: application/x-www-form-urlencoded
• new URLSearchParams({...}) instead of JSON.stringify({...})

References

This aligns with:

• OAuth 2.0 RFC 6749 Section 4.1.3 - token requests MUST use form-urlencoded
• Working implementations in other projects (letta-code, vybestack/llxprt-code)