fix(auth): url parse is not correct

[jokemanfire]

The format method is not good for url.
use authorization_endpoint: format!("{}/authorize", auth_base) will cause the result
"http://example.com//authorize"
replace with url parse will be great for avoiding this problem.
fix #397

Motivation and Context

fix url bug.

How Has This Been Tested?

No need UT.

Breaking Changes

NO

Types of changes

• 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

• 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

[Copilot]

Pull Request Overview

This PR fixes URL parsing issues in the OAuth authentication module by replacing string formatting with proper URL parsing to prevent double slashes in endpoint URLs.

• Introduces a helper function to properly construct endpoint URLs using URL parsing
• Replaces hardcoded "http://localhost" strings with a named constant
• Fixes the bug where format!("{}/authorize", auth_base) could produce URLs with double slashes

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}