Add assertion-based matplotlib tests for mpl_draw

[Rachel-Eva]

Fixes #1316

Adds a new TestMPLDrawAssertions test class that asserts actual
properties of the matplotlib output rather than just checking that
drawing doesn't crash. Tests are Linux-only as suggested by the
maintainer.

New tests cover:

• Node count reflected in collections

• node_list filtering

• Node color applied

• Node labels drawn as text

• Edge count

• Directed graphs produce arrow patches

• Edge labels drawn as text

• node_size=0 produces zero-size collection

• Empty graph produces no collections

• I ran rustfmt locally

• I have added the tests to cover my changes.

• I have updated the documentation accordingly.

• I have read the CONTRIBUTING document.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ IvanIsCoding
❌ kasryan
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[IvanIsCoding]

I will take a look at this more carefully later. But first things first: thanks for getting this started!

[IvanIsCoding]

@Rachel-Eva this LGTM but you need to sign the CLA before I am able to merge.

The email used for the commits also indicates another user: @kasryan. So either sign with that account or rewrite the commits to use a new email associated with your current account.

[Rachel-Eva]

Hi, I've been trying to rewrite the commits but ran into issues with the rebase. The commits attributed to @kasryan are mine, that account somehow got linked to my git config. Is there any way to resolve this on your end, or would it be easier if I close this PR and open a fresh one with the correct author info?

[IvanIsCoding]

Hi, I've been trying to rewrite the commits but ran into issues with the rebase. The commits attributed to @kasryan are mine, that account somehow got linked to my git config. Is there any way to resolve this on your end, or would it be easier if I close this PR and open a fresh one with the correct author info?

You can also edit the commits to update the email. And then do a force push. https://stackoverflow.com/questions/1186535/how-do-i-modify-a-specific-commit

As long as the CLA check pass you are fine. Up to you if you want a new PR or rewrite the PR.

[Rachel-Eva]

Hi @IvanIsCoding, I ran into some git history issues trying to rewrite the commits so I've opened a new PR #1567 with the correct author info.