fix(auth): support Auth0/OIDC namespaced custom role claims in JWT decoding

[MuzzaiyyanHussain]

Description

Fixes #1792 — Ensures Auth0 and other OIDC providers’ namespaced claims
(e.g. https://example.com/roles) are parsed into user_metadata.

Changes

• Updated decodeJWT() in helpers.ts to detect and flatten namespaced claims.
• Added test verifying roles are extracted correctly.

Impact

• Developers can now access Auth0-style custom claims via user.user_metadata.roles.

Checklist

• Build passes
• Added unit test
• Closes Supabase Rpc function sliently failing #1792

[coveralls]

coverage: 95.367% (+13.8%) from 81.58%
when pulling 537756b on MuzzaiyyanHussain:fix-auth0-namespaced-claims
into 1371057 on supabase:master.