Fixed issues in event subscription Implementation

[Ashi1993]

[OB3] Fixed issues in event subscription Implementation

This PR contains the modification done in event subscription handling.

Issue link: https://github.com/wso2-enterprise/financial-open-banking/issues/8320

Doc Issue: Optional, link issue from documentation repository

Applicable Labels: Spec, product, version, type (specify requested labels)

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Development Checklist

1. Built complete solution with pull request in place.
2. Ran checkstyle plugin with pull request in place.
3. Ran Findbugs plugin with pull request in place.
4. Ran FindSecurityBugs plugin and verified report.
5. Formatted code according to WSO2 code style.
6. Have you verify the PR does't commit any keys, passwords, tokens, usernames, or other secrets?
7. Migration scripts written (if applicable).
8. Have you followed secure coding standards in WSO2 Secure Engineering Guidelines?

Testing Checklist

1. Written unit tests.
2. Documented test scenarios(link available in guides).
3. Written automation tests (link available in guides).
4. Verified tests in multiple database environments (if applicable).
5. Verified tests in multiple deployed specifications (if applicable).
6. Tested with OBBI enabled (if applicable).
7. Tested with specification regulatory conformance suites (if applicable).

Automation Test Details

Test Suite	Test Script IDs
Integration Suite	TCXXXXX, TCXXXX

Conformance Tests Details

Test Suite Name	Test Suite Version	Scenarios	Result
Security Suite	VX.X	Foo, Bar	Passed

Resources

Knowledge Base: https://sites.google.com/wso2.com/open-banking/

Guides: https://sites.google.com/wso2.com/open-banking/developer-guides