Enforce oauth2.plugins as an allow-list for OAuth2 login providers

[Damans227]

Fixes #13862.

oauth2.plugins (default google,github, described as "List of OAuth plugins") was declared as a ConfigKey but never read anywhere. Actual provider availability was determined entirely by which Spring beans exist plus oauth2.plugins.exclude, so changing oauth2.plugins was a complete no-op, an admin could set it to github,keycloak and google would remain fully functional.

This wires it up as an allow-list: a provider must be named in oauth2.plugins to be usable.

• OAuth2AuthManagerImpl.getUserOAuth2AuthenticationProvider() now rejects any provider not in the configured list (covers both the oauthlogin and verifyOAuthCodeAndGetUser paths, since both resolve the provider through this method).
• OAuth2AuthManagerImpl.listUserOAuth2AuthenticationProviders() filters the same way, so listOauthProvider no longer reports a disallowed provider as enabled.
• Match is case-insensitive, consistent with how provider names are looked up elsewhere in this class.

Added test coverage for the allow-list check itself and for both call sites.

[Damans227]

@blueorangutan package

[blueorangutan]

@Damans227 a [SL] Jenkins job has been kicked to build packages. It will be bundled with no SystemVM templates. I'll keep you posted as I make progress.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.65%. Comparing base (e201213) to head (2ad135a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...pache/cloudstack/oauth2/OAuth2AuthManagerImpl.java	72.22%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #13866      +/-   ##
============================================
+ Coverage     19.64%   19.65%   +0.01%     
- Complexity    19785    19803      +18     
============================================
  Files          6368     6368              
  Lines        574889   574905      +16     
  Branches      70353    70358       +5     
============================================
+ Hits         112935   112996      +61     
+ Misses       449683   449635      -48     
- Partials      12271    12274       +3     

Flag	Coverage Δ
uitests	3.41% <ø> (ø)
unittests	20.93% <72.22%> (+0.01%)	⬆️

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[blueorangutan]

Packaging result [SF]: ✔️ el8 ✔️ el9 ✔️ el10 ✔️ debian ✔️ suse15. SL-JID 18841