Replies to threads will re-open issues that are fixed

### Describe the bug

The bot currently re-opens issues when a new reply comes in to the mailing list. This is okay for issues that where closed due to missing information, etc. However, it should not re-open issues that where fixed.

I propose we check if the close status of the issue if the state reason is `COMPLETED` we don't re-open, but instead reply with a standard reply stating this issue is now closed.

This should exclude email conversations with prodsec.

### Version

main

### Expected behavior

_No response_

### Actual behavior

_No response_

### How to Reproduce?

_No response_

### Anything else?

_No response_

Start at the bot's mailing-list reply handling entry point and trace how issue close status is checked before reopening. Verify the COMPLETED state reason produces a standard closed reply, while prodsec email conversations remain excluded; confirm the behavior with the relevant existing tests.

Written by the indexing model from the issue text.