fix: prevent OAuth parameter injection via returnTo (#2381)

[tusharpandey13]

Description

This PR addresses a security issue where OAuth parameters could be injected via the returnTo parameter in withPageAuthRequired.

Changes

• URL encode the returnTo parameter to prevent injection of OAuth parameters like &prompt=login
• Add comprehensive unit tests covering:
  • Basic returnTo encoding
  • OAuth parameter injection attempts
  • Special character handling
  • Function-based returnTo options

Related PRs

• Original security fix contributed by @MegaManSec in fix(app-router): encode returnTo in login redirect to prevent OAuth param injection #2381
• Adds missing unit test coverage as requested
• Uses properly signed commits
• Supersedes fix: prevent OAuth parameter injection via returnTo (#2381) #2411 (moved from fork to branch)

Testing

All existing tests pass, plus 3 new test cases specifically for this issue:

✓ should URL encode returnTo parameter to prevent OAuth param injection
✓ should URL encode returnTo with special characters
✓ should URL encode returnTo from function to prevent OAuth param injection

---

Note: This PR originally contained an attribution error in the commit message. The security issue was discovered and fixed by Joshua Rogers (@MegaManSec) in #2381. See correction comment below.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (0055cc4) to head (35eb321).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2413   +/-   ##
=======================================
  Coverage   90.39%   90.39%           
=======================================
  Files          39       39           
  Lines        4488     4488           
  Branches      912      911    -1     
=======================================
  Hits         4057     4057           
  Misses        425      425           
  Partials        6        6           

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[tusharpandey13]

I need to correct an attribution error in this PR. The original security fix was contributed by Joshua Rogers (@MegaManSec) in PR #2381, not "Simen A. W. Olsen" as incorrectly stated in the commit message.

We will update the changelog accordingly.