Refactor `syncService.start` method

[AnkushinDaniil]

• Extracted logic for getting the next block height into a separate method getNextHeight.
• Moved block processing logic into a new method processBlock to reduce the complexity of the start method.
• Improved error handling by using fmt.Errorf to wrap errors.
• Enhanced code readability and maintainability by breaking down large functions into smaller.

These changes aim to reduce complexity.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.45%. Comparing base (38638ca) to head (f758c3e). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2174 +/- ## ========================================== - Coverage 78.47% 78.45% -0.03% ========================================== Files 100 100 Lines 9240 9240 ========================================== - Hits 7251 7249 -2 Misses 1352 1352 - Partials 637 639 +2 ```

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