Add prompt=consent for OIDC offline_access scope

[dankelleher]

<!-- Provide a brief summary of your changes -->
Add automatic prompt=consent parameter when requesting offline_access scope in OpenID Connect flows. This ensures proper user consent for refresh token issuance as required by the OIDC specification.

Motivation and Context

<!-- Why is this change needed? What problem does it solve? -->
OpenID Connect specification requires that when requesting the "offline_access" scope (which grants refresh tokens), the authorization server must ensure the user explicitly consents to this access. By adding prompt=consent when offline_access is requested, we ensure OIDC-compliant authorization servers properly prompt users for consent.

Reference: https://openid.net/specs/openid-connect-core-1_0.html#OfflineAccess

How Has This Been Tested?

<!-- Have you tested this in a real application? Which scenarios were tested? -->

• Added unit test to verify prompt=consent is added when scope includes offline_access
• Verified existing authorization flows without offline_access are unaffected
• All existing tests pass

Breaking Changes

<!-- Will users need to update their code or configurations? -->
No breaking changes. This only affects flows that request the offline_access scope, and ensures they comply with OIDC requirements.

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

<!-- Go over all the following points, and put an x in all the boxes that apply. -->

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

<!-- Add any other context, implementation notes, or design decisions -->
This change specifically checks if the requested scope includes "offline_access" and appends prompt=consent to the authorization URL. This is a minimal change that improves OIDC compliance without affecting existing OAuth 2.0 flows.