fix(oauth2-redirect): guard window.opener for cross-origin flows

[yogeshwaran-c]

Description

When the OAuth2 authorization flow goes through a cross-origin redirect, browser policy can sever window.opener on the redirect tab. The current dev-helpers/oauth2-redirect.js (which webpack copies to dist/oauth2-redirect.html) accesses window.opener.swaggerUIRedirectOauth2 unconditionally, throwing TypeError: Cannot read properties of null (reading 'swaggerUIRedirectOauth2') and leaving the user staring at a blank redirect tab.

This PR adds a single guard at the top of run() that detects the missing opener and surfaces a clear instruction to the user instead of crashing silently.

Motivation and Context

Refs: #10786, #6150

Both issues report the redirect tab failing without a useful message in cross-origin OAuth flows. The fix is the minimum needed to convert the silent crash into actionable feedback; it does not alter the success path.

How Has This Been Tested?

• New Jest unit test test/unit/core/oauth2-redirect.js exercises the script under three scenarios:
  • window.opener is null → no throw, instruction text is shown
  • window.opener exists but swaggerUIRedirectOauth2 is missing → no throw, instruction text is shown
  • opener and oauth2 state are present → existing success path is preserved
• 3/3 unit tests pass locally:

  PASS test/unit/core/oauth2-redirect.js (47 s)
  Tests:       3 passed, 3 total
  

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.