core/parsigex: bind partial signature sender to share index

[KaloyanTanev]

Bind each partial signature to its authenticated sender's assigned share index on the runtime exchange path, reusing the VerifyPeerShareIdx check that already guards the DKG path. Without it, a cluster member can replay another peer's partial signature under that peer's share index; it is stored first and the honest peer's own partial signature is then rejected as mismatching, starving the aggregate below threshold and failing the duty.

category: bug
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 61.11111% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.47%. Comparing base (2ab39d6) to head (c3ef0d6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/app.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4705      +/-   ##
==========================================
- Coverage   65.52%   65.47%   -0.05%     
==========================================
  Files         246      246              
  Lines       30062    30069       +7     
==========================================
- Hits        19698    19689       -9     
- Misses      10363    10379      +16     
  Partials        1        1              

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