#501, added the endpoint for getting client's subscribed rules + testing

[AyushAdh1kari]

Changes

Added a new GET /rules/subscribed/{client_id} endpoint that returns only the rules a specific client is subscribed to. This includes a new get_client_rules method on RuleManager that looks up the client's subscribed rule IDs and resolves them to full Rule objects.

Notes

• Distinct from the existing GET /rules/{client_id} which returns all rules with subscription status — this endpoint returns only the subscribed subset

Test Cases

• Client with subscriptions returns only their subscribed rules (owned + explicitly subscribed)
• Non-existent / unsubscribed client returns an empty list

Checklist

It can be helpful to check the Checks and Files changed tabs.
Please review the contributor guide and reach out to your Tech Lead if anything is unclear.
Please request reviewers and ping on slack only after you've gone through this whole checklist.

• All commits are tagged with the ticket number
• No linting errors / newline at end of file warnings
• All code follows repository-configured prettier formatting
• No merge conflicts
• All checks passing
• Remove any non-applicable sections of this template
• Assign the PR to yourself
• No package-lock.json changes (unless dependencies have changed)
• Request reviewers & ping on Slack
• PR is linked to the ticket (fill in the closes line below)

Closes #501