feat: promote jax_zero_contour to default dependency (PyAutoLens#702)

[Jammy2211]

Summary

Part of promoting JAX to a default dependency of the stack (PyAutoLens#702). jax_zero_contour>=2.0,<3.0 moves from the [jax] extra into the base dependencies (marker-gated for Intel macOS, where jax has no wheels); jax and optax arrive via the autofit/autonerves chain. The [jax] extra becomes a declared no-op alias (PyAutoLens#687), and the intra-family floor comments record the follow-up floor bump to the first promoted releases.

The critical_curves_method='zero_contour' fallback warning (both plot/plot_utils.py and its util/ duplicate) no longer recommends the now-no-op [jax] extra — it points at pip install jax_zero_contour. Install docs (pip/conda) updated: pip install autogalaxy is the JAX-enabled command; Intel Mac NumPy-only fallback documented.

API Changes

None — no Python API changes. Packaging: jax_zero_contour moves into the default dependencies; [jax] becomes a no-op alias. One warning text updated.
See full details below.

Test Plan

• pyproject parses; markers verified (jax_zero_contour excluded on Intel macOS only)
• Edited files compile
• No-JAX CI leg (PyAutoHeart lib-tests.yml unittest-nojax) green once the Heart PR merges

Full API Changes (for automation & release notes)

Changed Behaviour

• pip install autogalaxy now installs JAX (via the family chain) and jax_zero_contour by default (except Intel macOS).
• The zero_contour fallback warning recommends pip install jax_zero_contour instead of the [jax] extra.

Migration

• Before: pip install autogalaxy[jax]
• After: pip install autogalaxy (the [jax] form still works — no-op alias)

Part of the six-repo JAX-default-dependency change: PyAutoLens#702.

Generated by the PyAutoLabs agent workflow.

[Jammy2211]

Workspace PRs: PyAutoLabs/autogalaxy_workspace#212, PyAutoLabs/HowToGalaxy#67