Bug: Assignment workflow treats mergeable PRs as merged

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

.

🔍 Issue Overview

The assignment workflow currently determines whether a pull request is merged by calling the GET /pulls/{pull_number}/merge endpoint and checking for a 204 status code.

However, this endpoint only indicates whether a pull request is mergeable, not whether it has actually been merged. As a result, open but mergeable pull requests can be incorrectly treated as completed.

🎯 Impact

This can lead to incorrect issue assignment behavior, where contributors may be allowed or blocked from taking new issues based on an inaccurate merge status.

✅ Expected Behavior

Only pull requests that have actually been merged should be treated as completed when validating active issue assignments.

🔧 Suggested Fix

Fetch pull request details using GET /pulls/{pull_number} and determine merge completion by checking the merged field in the response.

A pull request has already been opened to address this issue: #

(Replace after creating the PR.)

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