Refactor extrinsic receipt event parsing with shared helpers

[it-education-md]

Summary

This PR refactors extrinsic receipt event parsing for both the sync and async clients.

Problem

Before this change, the same event parsing logic lived in both sync_substrate.py and async_substrate.py.
Both implementations had become hard to read, harder to maintain, and more error-prone to change.
This cleanup was also already called out by inline TODOs in both files.

Solution

This PR is a behavior-preserving refactor to clean up receipt event parsing without changing the existing behavior contract.

The work was done in three steps:

1. Add dedicated sync and async unit tests first to lock down current receipt parsing behavior.
2. Extract the shared event parsing logic into utils/receipt.py.
3. Wire the shared helper functions into both implementations.

Test

In the dedicated sync and async unit test files, this PR adds essential but complete test coverage for the current receipt event parsing logic.

pytest -c tests/pytest.ini tests/unit_tests/sync/test_substrate_interface.py::TestExtrinsicReceiptProcessEvents -q
pytest -c tests/pytest.ini tests/unit_tests/asyncio_/test_substrate_interface.py::TestAsyncExtrinsicReceiptProcessEvents -q
pytest -c tests/pytest.ini tests/unit_tests -q

[thewhaleking]

Nice. I've always hated this. I'm going to modify this a bit and include it as part of ASI 2.0

[thewhaleking]

Tests blocked by #288

[it-education-md]

Thank you. Happy to contribute.