Add PR test CI + modernize test suite to pytest

[jack-champagne]

jl2py had no PR CI (only wheels.yml on tags). This adds a pull_request test
workflow and modernizes the test suite to idiomatic pytest.

CI (.github/workflows/test.yml): on PR, set up Julia 1.12.6 + Python 3.12,
make libjl2py, pip install numpy pytest, run pytest -v out/jl2py/tests.

pytest conversion (the suite was script-style with per-file setup()/main()):

• tests/conftest.py — session-scoped jl fixture (the test functions already take jl).
• Removed the setup()/main()/__main__ boilerplate from every file.
• test_struct.py — its struct definitions moved into a module-scoped autouse fixture.
• test_array.py — the 3 memory-wrap tests now skipif cleanly when the experimental
  libjulia-internal build isn't present (tracked in CI: add a JL2PY_INTERNAL build matrix cell for Memory wrap/alloc #8).
• New: test_identity.py (pointer-identity round-trip, PRD §4.3) and test_exceptions.py
  (typed error mapping — BoundsError→IndexError, DomainError→ValueError, etc.).

Local run (Julia 1.12.6): 76 passed, 3 skipped, 0 failed.

Follow-ups filed: #5–#10.

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