refactor: add get_available_parallelism function

[alan910127]

Which issue does this PR close?

Closes #13591.

Rationale for this change

In #13579 (review), it was suggested that the repeated use of std::thread::available_parallelism with fallback logic could be refactored for clarity and to reduce duplication. Extracting this logic into a utility function improves code readability and reduces the likelihood of errors.

What changes are included in this PR?

This PR introduces a new utility function:

pub fn get_available_parallelism() -> usize;

It replaces the following repeated code snippet:

std::thread::available_parallelism().unwrap_or(NonNull::new(1).unwrap()).get()

with a call to the new function:

get_available_parallelism()

Are these changes tested?

No tests are added for this change as it is a straightforward refactor of existing functionality. However, if reviewers feel this requires testing, I am happy to add them.

Are there any user-facing changes?

No.

[findepi]

The clippy job failed. This is probably nothing wrong with this PR, see #13597.

[comphead]

Thanks @alan910127 for your first contribution.
Please take care on clippy. I feel the PR is good.

[alan910127]

Hi @comphead, thanks for your review! I just checked the pipeline logs, the clippy errors are not related to my changes. Perhaps it's what @findepi mentioned in the previous comment.

[comphead]

@alan910127 please rebase from the latest main

[alan910127]

@comphead Rebased.

[comphead]

lgtm thanks @alan910127

[comphead]

Since this is a first time contribution I'll be waiting for another review before merging it in

[jonahgao]

LGTM, thanks @alan910127 . I merged main, so that this PR can be checked by the new version of clippy.