fix(python): inconsistent struct hash calculation between Java and Python

[LouisLou2]

What does this PR do?

Problem

The struct hash calculation implementations in Java and Python are inconsistent. When classinfo is None in the Python implementation, the method returns early without updating the hash value, whereas the Java implementation continues with a default hash value of 0.

Solution

Modified the visit_customized method in the Python implementation to initialize hash_value to 0 before checking if classinfo is None, ensuring the hash computation continues regardless of the classinfo status. This approach aligns with the Java implementation behavior.

Changes:

• Initialize hash_value to 0 at the beginning of the method
• Replace the early return with a conditional assignment to hash_value
• Always call _compute_field_hash at the end of the method

Testing

Verified the fix by running the CrossLanguageTest.java tests that were previously failing due to this issue. The struct hash values now match between Java and Python implementations.

Related issues

• [Python] Inconsistent struct hash calculation between Java and Python implementations #2107

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

[chaokunyang]

Code lint check failed, please use black to format _struct.py

[chaokunyang]

Looks great