Add pytato.array_api: Python Array API conformance namespace + test infrastructure

[Copilot]

Implements the Array API 2023.12-compatible namespace (pytato.array_api) and wires up the array-api-tests conformance suite against it.

How it works

Each operation in pytato.array_api wraps inputs in pytato DataWrapper nodes, builds the pytato expression graph, then immediately evaluates via the new NumpyPythonTarget — exercising pytato's full codegen pipeline while returning concrete numpy results the conformance tests can inspect.

import pytato.array_api as xp

x = xp.asarray([1.0, 2.0, 3.0])
y = xp.sum(x)            # builds pt.sum() graph, evaluates via numpy codegen
z = xp.matmul(xp.ones((2, 3)), xp.ones((3, 2)))  # pt.matmul → numpy

Changes

• pytato/array_api.py — new module implementing the full Array API 2023.12 surface: creation, element-wise, reductions, manipulation, searching/sorting, set functions, linear algebra, indexing, __array_namespace_info__, etc.
• pytato/target/python/__init__.py — adds NumpyPythonTarget, a concrete NumpyLikePythonTarget subclass that generates code targeting numpy (used by array_api.py for eager evaluation).
• pytato/target/python/numpy_like.py — fixes ProductReductionOperation codegen mapping from removed numpy.product → numpy.prod (NumPy 2.0 compatibility bug).
• .github/workflows/array_api.yml — CI job that clones array-api-tests at HEAD and runs the conformance suite against pytato.array_api.
• array-api-tests.xfails.txt — expected failures: missing optional extensions (linalg, fft) and scalar weak-typing differences (Python int scalar + integer array dtype promotion deviates from Array API spec).

Current conformance status

With --max-examples=50: 452 pass, 37 fail (all failures are the unimplemented linalg/fft optional extensions listed in xfails.txt), 97 skipped.

[alexfikl]

@inducer This should probably make it to main first, since np.product no longer exists 😁