Only enable Memray options when the pytest plugin is installed - #5307

The Python 3.15 and brotlipy jobs in CI run 38137864167 stop before collecting tests with unrecognized arguments: --memray --hide-memray-summary.

dev-base excludes pytest-memray when python_full_version >= '3.15', but tests_impl() enables its CLI options for final, GIL-enabled CPython on non-Windows platforms without checking whether the plugin was installed. The same noxfile blob is present on current main (5ebe9bf9f691646dcabdb259ffabfc6108ded5e8) and the failing run's head.

Check importlib.util.find_spec('pytest_memray') in the session's interpreter after installing dependencies, before adding Memray options. Keep the existing implementation, release-level, free-threading and Windows restrictions. This follows the installed dependency selection without duplicating its Python version ceiling. Coverage invocation, settings and thresholds are unchanged.

Validation on macOS:

• Before the fix, a mock Nox session with final CPython and no plugin executed the generated coverage/pytest command against a real smoke test: pytest exited 4 with the same unrecognized arguments. After the fix, it exited 0.
• Six mock-session cases passed: missing plugin, installed plugin, prerelease, free-threaded CPython, PyPy and Windows. Assertions also checked both Memray flags and retention of the coverage command/environment.
• uvx nox -s test-3.14 -- test/test_util.py: 595 passed, 6 skipped, with pytest-memray installed and enabled.
• After removing pytest-memray and memray from that disposable environment, uvx nox -r --no-install -s test-3.14 -- test/test_util.py: 595 passed, 6 skipped, with neither Memray option passed.
• uvx nox -s test-3.15 -- test/test_util.py: 595 passed, 6 skipped on Python 3.15.0rc2.
• uvx nox -P 3.15 -s test_brotlipy -- test/test_response.py -k brotli: 3 passed, 13 skipped, 211 deselected on Python 3.15.0rc2 (the normal warning about brotlipy lacking decompression-bomb protection was emitted).
• Black, flake8 (including flake8-2020), isort, git diff --check, and targeted mypy -m noxfile passed.
• The full uvx nox -s mypy command failed on an unchanged annotation in src/urllib3/connection.py:150 (Final can be only used as an outermost qualifier).

Limits: no full test suite, aggregated coverage check, or Linux Python 3.15 final run was performed locally. The final-release missing-plugin branch was exercised with a mock session and real pytest; the available local Python 3.15 interpreter is an RC. The regression harness was temporary, not added to the repository's test suite.

Checked open PRs/issues for Memray, Python 3.15, brotlipy and the unrecognized-arguments error. The Memray search hits (#5226, #5237, #5162) do not change this gating; #5263 retains it. No duplicate fix was found.

This change and its validation were prepared with OpenAI Codex assistance. It is independent of #5286 and changes no timeout behavior.