chore(node-api): bound validator index user input on proof apis

[bar-bera]

There's a misleading #nosec statement which is only true if the user provided input is actually bounded to the max registry size.

Not causing any intentional or unintentional damage. Just adding the bounds for correctness.

[Copilot]

Pull request overview

This PR hardens the node-api Merkle proof helpers by bounding user-supplied validator indices to the maximum validator registry size, making the existing #nosec G115 justifications for uint64 -> int conversions accurate.

Changes:

• Add validatorIndex bounds checks (< constants.ValidatorsRegistryLimit) before computing offsets used in int(...) conversions.
• Update #nosec G115 annotations/comments to clarify why the int conversion is safe (on 64-bit builds) given the new bounds.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
node-api/handlers/proof/merkle/validator_pubkey.go	Adds validator index bounds check before computing the validator gIndex offset; adjusts #nosec rationale.
node-api/handlers/proof/merkle/validator_credentials.go	Adds validator index bounds check before computing offsets used in gIndex calculation; adjusts #nosec rationale.
node-api/handlers/proof/merkle/validator_balance.go	Adds validator index bounds check and updates #nosec rationale for gIndex computation (but one comment needs correction).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.24%. Comparing base (ffe5e16) to head (a11d9ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3103      +/-   ##
==========================================
+ Coverage   61.21%   61.24%   +0.03%     
==========================================
  Files         369      370       +1     
  Lines       18925    18941      +16     
==========================================
+ Hits        11584    11600      +16     
  Misses       6382     6382              
  Partials      959      959              

Files with missing lines	Coverage Δ
...ode-api/handlers/proof/merkle/validator_balance.go	66.31% <100.00%> (+0.72%)	⬆️
...api/handlers/proof/merkle/validator_credentials.go	60.29% <100.00%> (+1.83%)	⬆️
...pi/handlers/proof/merkle/validator_index_bounds.go	100.00% <100.00%> (ø)
node-api/handlers/proof/merkle/validator_pubkey.go	61.19% <100.00%> (+1.81%)	⬆️

... and 1 file with indirect coverage changes

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