fix: implement public_keys filtering in ListAccounts

[radik878]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

Implemented filtering by public_keys before pagination while preserving original key indices for correct HD derivation paths.
Add TestServer_ListAccounts_FilterAndPagination to verify filtering and pagination behavior.

Which issues(s) does this PR fix?

• Previously, GET /v2/validator/accounts parsed and validated query public_keys but never used them to filter results, leaving dead code and confusing behavior.
• This was inconsistent with neighboring handlers (e.g., GetValidators, GetValidatorBalances, GetValidatorPerformance) where public_keys directly affects the response.

Other notes for review

Pagination and TotalSize now operate on the filtered set; all=true returns all filtered accounts; empty filter results return an empty list without pagination errors.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.

[CLAassistant]

All committers have signed the CLA.

[james-prysm]

Seems like a lot of stuff is failing on linting

[radik878]

Seems like a lot of stuff is failing on linting

Ohhhh yes... it's my bad, copy-paste mistake

[radik878]

Seems like a lot of stuff is failing on linting

corrected, sorry for inconvenience