fix: blockhash syncing

[FedericoAmura]

Description

Added some safety measures when syncing blockhash. More specifically:

• verify fetch response from block-indexer is successful to avoid using error responses with undefined blockhashes
• use previous-to-last blockhash from public providers on fallback to avoid using a blockhash not yet propagated to nodes
• use block timestamp instead of user one as that is the one that defines if blockhash is still valid

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added unit tests for LitCore
• CI

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Ansonhkg]

LGTM - awaiting test results since our CI is broken atm.