Fix Python object API Pack for fixed arrays of structs

[momomuchu]

Bug

The Python object API Pack() code generator accessed a fixed array of structs as a single object instead of iterating its elements. For a struct field that is a fixed array of structs, Pack() raised AttributeError or produced wrong output.

Root cause

In the Python code generator (src/idl_gen_python.cpp), the object API Pack() code emitted a flat field access (self.d.a) for a fixed array of structs, instead of iterating the array elements.

Fix

The generator now emits recursive helpers: it walks struct fields carrying an array context stack, and for a fixed array of structs it wraps each leaf field in one list comprehension per array depth, so nested fixed arrays of structs pack correctly at any depth. Scalar arrays and non array fields are unchanged. The committed generated Python test files are regenerated to match the fixed flatc output.

Tests

Added an object API Pack test for a fixed array of structs (Pack then read back, asserting nested values). The full Python test suite passes via PythonTest.sh. Reverting the generator change makes the test fail with AttributeError.

Fixes #9061

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