Override hashCode on SProductField to match its equals

[adamw]

Fixes #5220.

SProductField overrode equals() to compare name and schema, but lacked a matching hashCode() implementation, violating the Java contract that equal objects must have equal hash codes. This causes issues when SProductField instances are used in Sets or HashMaps.

Adds hashCode() override derived from the same fields used in equals comparison. Includes test verifying equal fields have matching hash codes and work correctly in collections.

Closes #5220.

[adamw]

The failure is clear and consistent across all platforms. Here's the summary:

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The test should have matching hashCodes for equal SProductFields fails across all platforms (JVM, JS, Native) for all Scala versions (2.12, 2.13, 3):

SchemaTest.scala:280 — X was not equal to Y

Examples:

• JVM 2.13: 1686160193 was not equal to 1729653852
• JS 2.13: 10 was not equal to 11
• Native 3: 1895159083 was not equal to 1895158859

Root cause: SProductField has a custom equals implementation but no hashCode override, violating the equals/hashCode contract. Two SProductField instances that are equal per equals produce different hash codes. This PR adds the failing test to document the problem, but does not yet add the hashCode implementation to fix it.