refactor: improve type safety in OIDC module

[dirkwa]

Improve type safety in OIDC module

Summary

• Replace [key: string]: any with [key: string]: unknown in SecurityConfigForOIDC interface
• Fix potential undefined access in decodeIdToken function

Details

Stricter index signature typing (oidc-admin.ts)

Changed the index signature from any to unknown, which enforces proper type checking when accessing dynamic properties. This eliminates the need for the eslint-disable comment and aligns with TypeScript strict mode best practices.

Safer array access (user-info.ts)

The decodeIdToken function now explicitly checks that the payload part exists before using it. While the length check already ensures 3 parts, TypeScript's strict mode requires this additional guard since array index access can return undefined. This makes the code more robust and satisfies strict null checks.