Use point.is_zero() for allocating infinity in SW curves

[Pratyush]

Description

Needed for arkworks-rs/algebra#639, and is anyway the right thing to do.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[Copilot]

Pull Request Overview

This PR refactors how the “infinite” flag is computed for Short Weierstrass curve points by using point.is_zero() instead of a direct point.infinity field.

• Replaces point.infinity with point.is_zero() in the generic SW impl
• Makes the same change in the BLS12-specific SW allocation
• This ensures consistency with the updated group API where the identity is checked via is_zero()

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/groups/curves/short_weierstrass/mod.rs	Swapped point.infinity for point.is_zero()
src/groups/curves/short_weierstrass/bls12/mod.rs	Updated the closure mapping to use g.is_zero()

Comments suppressed due to low confidence (1)

src/groups/curves/short_weierstrass/mod.rs:168

• Add a unit test for AffineVar::new that allocates both a finite and the point-at-infinity case, verifying that point.is_zero() correctly drives the Boolean flag.

            let infinity = Boolean::constant(point.is_zero());

[Copilot]

[nitpick] Consider updating the namespace label ("inf") or the variable name infinity to something like is_zero or is_identity to align with the is_zero() check and reduce potential confusion.