[BUG] Fix unsafe PluginConfigVariable casts in AtlassianOauthConfig #6874

Describe the bug
AtlassianOauthConfig.java lines 153-154 perform unsafe (String) casts on PluginConfigVariable.getValue() which returns Object. If the underlying value is not a String, this throws ClassCastException at runtime.

  this.accessToken = (String) oauth2Config.getAccessToken().getValue();
  this.refreshToken = (String) oauth2Config.getRefreshToken().getValue();

To Reproduce

1. Configure Confluence/Jira source with OAuth2 authentication
2. If the PluginConfigVariable implementation wraps the value in a non-String type, renewCredentials() in AtlassianOauthConfig throws ClassCastException

Expected behavior
Should use instanceof String check before casting, or use value.toString() as fallback, to handle any Object type safely without crashing.

Screenshots
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Additional context
Found during review of #6856. Kept out of scope to avoid mixing OAuth2 changes with bearer token changes. The fix should follow the same pattern established in AtlassianBearerTokenAuthConfig.getBearerToken() after #6856