fix: patch the query collision bug in multipoint opening

[guorong009]

Description

The multipoint opening argument is used to efficiently evaluate multiple polynomial commitments at
multiple points. In the implementation, a map is used to track queries: (Commitment, QueryPoint) =>
Value. Here, the key is a tuple of the polynomial commitment and the query point, and the value is the
evaluation of the polynomial at that point. A “query collision” occurs when two independent queries
have the same key—meaning they refer to the same commitment and the same evaluation point. When
this happens, one evaluation silently overwrites the other in the map, causing one to be ignored during
verification. This is a critical flaw: a malicious prover can exploit this to forge a proof, since one of the
evaluations will not be checked in the polynomial commitment opening.

Changes

• catch the mismatching query evaluations in construct_intermediate_sets func in both shplonk and gwc schemes
• add the tests exercising the bug

NOTE

The patch was co-authored by Jack (str4d) Grigg and Sean Bowe, and reviewed by Daira-Emma and Edu. The bug was reported by Suneal from zkSecurity. The patch is ported from here

[guorong009]

@mratsim can you also review the PR? Thanks in advance!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.14286% with 11 lines in your changes missing coverage. Please review.

Project coverage is 82.89%. Comparing base (495cbcd) to head (acb91eb).

Files with missing lines	Patch %	Lines
halo2_backend/src/poly/kzg/multiopen/gwc/prover.rs	33.33%	4 Missing ⚠️
...2_backend/src/poly/kzg/multiopen/shplonk/prover.rs	33.33%	4 Missing ⚠️
halo2_backend/src/poly/kzg/multiopen/shplonk.rs	87.50%	2 Missing ⚠️
halo2_backend/src/poly/multiopen_test.rs	98.80%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
+ Coverage   82.82%   82.89%   +0.06%     
==========================================
  Files          76       76              
  Lines       16988    17110     +122     
==========================================
+ Hits        14071    14183     +112     
- Misses       2917     2927      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[guorong009]

@CPerezz @davidnevadoc
Can you also take a look at the PR if you have some time?

[ed255]

LGTM! Great job porting these patches to our fork :)

[mitsu0925c74]

Great job @guorong009 💪