Refactor subscription/unsubscription flows

[ayeshLK]

Purpose

$subject

Fixes: #7586

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[sonarqubecloud]

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]

Codecov Report

Attention: Patch coverage is 83.57143% with 23 lines in your changes missing coverage. Please review.

Project coverage is 87.80%. Comparing base (def0849) to head (3e30d3d).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
ballerina/subscription.bal	83.13%	14 Missing ⚠️
ballerina/http_service.bal	83.33%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1080      +/-   ##
============================================
- Coverage     88.19%   87.80%   -0.40%     
  Complexity       37       37              
============================================
  Files            13       13              
  Lines           788      787       -1     
  Branches        221      224       +3     
============================================
- Hits            695      691       -4     
- Misses           90       93       +3     
  Partials          3        3              

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