Fix sender_id logic in messages.py (#1151)

[choutankumarnaik-coder]

What does this PR do, and why?

Fixes the sender_id identification logic in messages.py for issue #1151.

External discussion & connections

• Fully fixes Two users with the the same name appear as the same person #1151
• Discussed in #GSoC stream - Topic: GSoC 2026 - Choutankumar Naik

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes

[zulipbot]

@choutankumarnaik-coder You've referenced #1151 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #1151..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

In addition, please make sure your commit structure and messages follow all guidelines.

[choutankumarnaik-coder]

I used AI to help brainstorm the logic, but I have manually re-written the implementation to use .get() and verified that all formatting aligns with Zulip’s style guidelines.