fix: blockhash syncing v6

[FedericoAmura]

Description

This is just a backport of #727 for our v6 SDK

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
• Local run of CI tests

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

[FedericoAmura]

Local CI run

- testExecuteJsSignAndCombineEcdsa - Passed (7196.49 ms)
- testUseEoaSessionSigsToExecuteJsSigning (Passed in 3987.37 ms)
- testUseEoaSessionSigsToPkpSign (Passed in 4129.60 ms)
- testUsePkpSessionSigsToExecuteJsSigning (Passed in 5326.51 ms)
- testUsePkpSessionSigsToPkpSign (Passed in 10112.03 ms)
- testUseValidLitActionCodeGeneratedSessionSigsToExecuteJsSigning (Passed in 5246.67 ms)
- testUseValidLitActionCodeGeneratedSessionSigsToPkpSign (Passed in 6190.24 ms)
- testDelegatingCapacityCreditsNFTToAnotherWalletToExecuteJs (Passed in 9009.74 ms)
- testEthAuthSigToEncryptDecryptString (Passed in 3842.77 ms)
- testExecuteJsBroadcastAndCollect (Passed in 4434.71 ms)
- testExecutJsDecryptAndCombine (Passed in 4028.52 ms)
- testExecuteJsSignAndCombineEcdsa (Passed in 7196.49 ms)

Test Report: 11 test(s) passed, 0 failed, 0 skipped.

[Ansonhkg]

LGTM

nit: The branch version should be v6.11.3

[FedericoAmura]

LGTM

nit: The branch version should be v6.11.3

Rebasing was not trivial, other commits appeared and also had to recreate the v6 master as that was deleted. Created everything in #732 and will be closing this, that was easier