fix: improve error messaging for invalid redirect_uri OAuth error

[quantcode-agent]

Summary

• Detects invalid_redirect_uri OAuth errors specifically (by checking the error code and description)
• Replaces the generic "Authorization Error" message with a clear, actionable message that explains the issue and suggests using the Manual Token option
• Provides a clickable link to https://dashboard.quantcdn.io/profile where users can generate a personal access token

Changes

src/Controller/OAuthController.php

• Added use Drupal\Core\Render\Markup import
• Added detection logic for redirect_uri-related OAuth errors (stripos on error description + exact match on error code)
• When a redirect_uri error is detected, displays: "Your MCP client's redirect URI isn't registered for this OAuth app. Try the Manual Token option instead — generate a personal access token at [link] and paste it into your client."
• Non-redirect_uri errors continue to show the existing generic message
• Wrapped HTML message in Markup::create() to ensure the link renders as clickable HTML

Assumptions

• The OAuth error parameters come via query string (error and error_description) as per OAuth 2.0 spec
• The error code invalid_redirect_uri or description containing "redirect_uri" are sufficient signals to detect this condition
• The hardcoded URL https://dashboard.quantcdn.io/profile is the correct location for personal access token generation
• Using Markup::create() is appropriate here since the URL is hardcoded (not user-supplied), so there's no XSS risk

Closes #1