Fix user name association in PostAuthAssociationHandler for configured idp subject identifier

[kayathiri4]

Fixes https://github.com/wso2/product-is/issues/21465

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 4.22535% with 68 lines in your changes missing coverage. Please review.

Project coverage is 41.02%. Comparing base (85e4e76) to head (12227df). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...impl/JITProvisioningPostAuthenticationHandler.java	9.09%	28 Missing and 2 partials :warning:
.../authentication/framework/util/FrameworkUtils.java	0.00%	27 Missing :warning:
...ndler/request/impl/PostAuthAssociationHandler.java	0.00%	10 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6125 +/- ## ============================================ + Coverage 40.77% 41.02% +0.24% - Complexity 14474 14654 +180 ============================================ Files 1772 1775 +3 Lines 117761 118417 +656 Branches 19116 19216 +100 ============================================ + Hits 48019 48579 +560 - Misses 62451 62536 +85 - Partials 7291 7302 +11 ``` | [Flag](https://app.codecov.io/gh/wso2/carbon-identity-framework/pull/6125/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/wso2/carbon-identity-framework/pull/6125/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `25.45% <4.22%> (+0.48%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2#carryforward-flags-in-the-pull-request-comment) to find out more.

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