[BUG]: Fix reloaded sphere crashing due to chunking

[jp-duminy]

PR Summary

This PR fixes a discovered issue wherein a reloaded sphere dataset produces a TypeError if the sphere is chunked. This arises from an oversight in the chunking implementation: an applied boolean mask was sized to a chunk, whereas the dataset it was applied to was sized to the original file. This therefore produces an incompatible shape error from fancy indexing. The fix is minor and I have added comments to explicitly point out the different file shapes to hopefully prevent such an oversight occurring in future.

The second commit of the PR adds a small unit test to check this behaviour. It errors on the current yt version, but now passes following the changes. As part of this test we also demand the reloaded sphere dataset reproduces the data before it was saved; this also passes.

The issue was first described in #5576.

AI Disclosure

I used AI for (leaving both boxes unchecked certifies that no AI was used in these changes):

• low level assistance: code-completion, syntax lookup, explaining existing code.
• substantive generation: substantive portions of code and/or documentation were written by AI then edited or submitted as is. This includes copying code from an AI chat or agentic coding. I have read and can explain every changed line.

I used the following AI tools:

Claude (Anthropic), Claude Opus 5.5

The source code modification in the first commit is written by me; I had Claude take a look to catch anything I might have missed. The corresponding test was then written by me.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.
• AI Disclosure completed if AI tools were used.

[welcome]

Hi! Welcome, and thanks for opening this pull request. We have some guidelines for new pull requests, and soon you'll hear back about the results of our tests and continuous integration checks. Thank you for your contribution!

[jp-duminy]

@brittonsmith a fun side quest on the project.

[chrishavlin]

thanks for the PR @jp-duminy ! the failing test is unrelated and I think will be fixed by a merge with main

[chrishavlin]

@jp-duminy just did the merge for you and pushed it up FYI.

[chrishavlin]

The change looks good, but the test needs an update (see comments). Note, pull the branch locally before making changes since I merged your branch with main for you.

[chrishavlin]

assuming tests pass again, this is good to go! Thanks for the issue and fix @jp-duminy !

[welcome]

Hooray! Congratulations on your first merged pull request! We hope we keep seeing you around! 🎆

[jp-duminy]

@chrishavlin thank you for walking me through this one, and I'm glad I could contribute. Have a lovely weekend.