gh-94937: Allow len() to return large Python lengths

[cjc0013]

This implements the len() fallback discussed in gh-94937 while keeping the existing Py_ssize_t fast path.

The change adds an internal object-returning length helper. It first uses PyObject_Size() for the normal C protocol path, then falls back on OverflowError to the type-level __len__ special method and validates the result with PyNumber_Index() plus the existing non-negative check.

range now exposes a METH_COEXIST __len__ method that returns its stored Python integer length, so large ranges can participate in the fallback without changing PyObject_Size() or the C length protocol.

The optimized len() bytecode paths now use the same helper as the builtin, so callers such as random.choice() and random.sample() see the same behavior.

Tests and docs are updated for:

• pure Python __len__ values larger than sys.maxsize
• special-method lookup ignoring instance-level __len__
• large range lengths
• random.choice() and random.sample() over ranges larger than sys.maxsize

Tested with:

• PYTHONPATH=./Lib:./Modules ./python.exe -m unittest -v test.test_builtin.BuiltinTest.test_len test.test_range.RangeTest.test_large_range test.test_random.MersenneTwister_TestBasicOps.test_choice test.test_random.MersenneTwister_TestBasicOps.test_sample_inputs
• PYTHONPATH=./Lib:./Modules ./python.exe -m test test_builtin test_range test_random

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± library/functions.html
± library/stdtypes.html
± reference/datamodel.html
± whatsnew/changelog.html