fix(python): make fixed array metadata methods static

[Pybsama]

Summary

• generate fixed-array Length and IsNone accessors as parameterless static methods
• keep dynamic-vector accessors as instance methods
• regenerate Python runtime outputs and stubs, with regression coverage for both class and instance calls

Root cause

Fixed-length array metadata depends only on schema constants, but the Python generator emitted these accessors with self in both .py and .pyi output.

Testing

• ./tests/PythonTest.sh
• cmake --build build --target flattests -j 4 && ./build/flattests
• focused unittest for fixed-array class and instance calls
• direct validation of all generated fixed-array accessors and an unchanged dynamic-vector failure mode
• sh scripts/clang-format-git.sh
• git diff --check

Fixes #9069

[google-cla]

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[Pybsama]

@googlebot rescan

[Pybsama]

Fresh rebase/validation follow-up (2026-08-29):

• Rebased the existing single patch onto current master (5761d6e6); the stable patch-id is unchanged. The new PR head is f6213ec47c8d9a6ae64e5d0e0e960e2b0b1a6c27, ahead 1 / behind 0, with the same 8-file scope.
• Fresh out-of-source build: flatc and flattests built successfully, and flattests reported ALL TESTS PASSED.
• ./tests/PythonTest.sh passed both multi-file and one-file generation modes (128 Python + 60 flexbuffers tests in each mode, 376 executions total). A direct boundary check also passed 44 class/instance calls across all 11 generated fixed-array fields and confirmed that a dynamic vector Length accessor still requires an instance.
• The six changed .py / .pyi artifacts compile, scripts/clang-format-git.sh made no changes, git diff --check is clean, and an independent review found 0 Critical / 0 Important / 0 Minor issues.

The fresh CI and OSS-Fuzz workflow runs currently show action_required with zero jobs, so no upstream language test has executed yet; they need maintainer approval for this external-fork head. The label, CLA, and check-changes checks succeeded, while the four zizmor fan-out jobs were skipped.