[Python] Fix FlexBuffers typed vector float conversion

[winklemad]

Summary

• Fix Ref.AsFloat for typed and fixed typed vectors by treating their type checks as properties.
• Use AsFixedTypedVector when reading fixed typed-vector length.
• Add regression coverage through the public Builder, Finish, and GetRoot APIs.

Problem

IsTypedVector and IsFixedTypedVector are properties, but AsFloat called them as functions. Valid typed vectors therefore raised TypeError. The fixed-vector branch also referenced the nonexistent FixedTypedVector accessor.

Testing

• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=python python3 -m unittest tests.py_flexbuffers_test.DecoderTest.test_typed_vectors_as_float
• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=../python python3 py_flexbuffers_test.py
• Both generated-code modes of py_test.py — 127 tests passed each.
• git diff --check

The wrapper suites tests/PythonTest.sh and tests/TestAll.sh, which require CMake and a built flatc for their Python stage, were not run locally. The relevant Python runtime suites were run directly.

[winklemad]

Rebased onto current master to clear the stale merge base — CI is green and there are no conflicts now.

Quick recap for whoever picks this up: Ref.AsFloat calls IsTypedVector() and IsFixedTypedVector() as methods, but both are @property, so AsFloat raises TypeError: 'bool' object is not callable on any typed or fixed typed vector. It also references a non-existent FixedTypedVector attribute instead of AsFixedTypedVector. The change is two source lines plus a regression test that fails on master (the TypeError) and passes with the fix; the full py_flexbuffers_test.py suite stays green (61 passed).

Happy to adjust anything if you'd like it shaped differently.