Fix NNX axis metadata under Linen scan and vmap

[Junyi-Zheng]

What does this PR do?

Fixes #5589.

NNXMeta.add_axis and remove_axis currently do nothing, so Linen scan and vmap can change variable shapes without updating axis metadata or invoking axis hooks. This can produce incorrect partition specifications and sharding errors.

This change:

• Updates axis metadata through the NNX SPMD helpers without reinitializing or resharding values.
• Handles empty metadata tuples and negative mapped axes.
• Preserves the underlying variable type when converting HiJAX variables.
• Adds regression tests for metadata, hooks, scan/vmap, sharding, JIT, and gradients.

Validation

• All 72 bridge tests passed with JAX 0.11.1, including 36 new regression cases.
• Full core tests ran on Python 3.12–3.14. Each run had one failure in test_tabulate_enum, also reproduced on the unmodified baseline.
• All pre-commit hooks and library pytype checks passed. Full-library mypy errors were identical to the baseline.
• Documentation builds, doctests, and built-wheel smoke tests passed.
• Validation used macOS CPU, including four virtual CPU devices. GPU/TPU execution remains unverified.

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• This change is discussed in a Github issue: NNXMeta.add_axis / remove_axis are no-ops, silently dropping axis metadata under Linen scan / vmap #5589.
• The documentation and docstrings adhere to the documentation guidelines.
• This change includes necessary high-coverage tests.