THRIFT-5943: optimize python C extension readStruct for nested structs

[markjm]

In this area of code, there already exists an optimization where we skip calling klass(**kwargs) on the top-level struct and instead init an empty object then PyObject_SetAttr the properties onto it.

This was done because Python's keyword argument matching is O(n) string comparisons per argument, making this expensive for structs with many fields.

However, nested structs which are decoded within the C extension did not get this same optimization, causing them to go through the slow class initialization path.

In this change, for mutable nested structs, we create the instance up front with a no-arg constructor (klass()) and set decoded fields directly via PyObject_SetAttr (the same way we fast-path the top-level struct).

Immutable structs (TFrozenBase subclasses) continue to use the kwargs path since their generated __setattr__ blocks attribute mutation.

Immutability is detected via (1) PyObject_IsSubclass against TFrozenBase with the class reference cached in a function-local static or (2) and overridden __setattr__.

Benchmarks show up to 3.6x speedup for deeply nested struct hierarchies, with no impact on flat structs. I had Claude generate me some benchmarks to showcase performance in different nested scenarios:

https://gist.github.com/markjm/e8610298d30d32869f46cf521272c85c

Checklist

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• [n/a] If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[markjm]

Regarding:

Note the test change. I asked about potentially changing this to look more like what the codegen produces here https://github.com/apache/thrift/pull/3349/changes#r3026288637. Currently, this fails because its not marked as frozen but has a erroring setattr

the test suite alerted me to another mechanism which something could look frozen which i needed to handle by checking 2 options. with that I dont need to modify the tests

   // Immutable structs are produced by two codegen paths:
    //   1. "frozen2" mode: classes inherit from TFrozenBase
    //   2. "python.immutable" annotation: classes get a __setattr__ that raises TypeError

[markjm]

thanks for kicking the builds @fishy - all green 🕺