Skip invalid L-BFGS curvature updates

[Trighap52]

Fixes #1708.

Summary

• Skip L-BFGS memory updates when the curvature pair has non-positive s^T y.
• Avoid writing negative inverse-curvature weights into weights_memory.
• Add regression coverage for the backtracking line-search reproducer from the issue.
• Document the skip behavior for lbfgs and scale_by_lbfgs.

Why

When backtracking/Armijo line search is used, L-BFGS can observe a parameter/gradient pair that does not satisfy the positive-curvature condition. Storing that pair makes weights_memory negative, which can break the inverse Hessian approximation. This change leaves such slots as zero so the two-loop recursion ignores them.

Tests

• pre-commit run --files optax/_src/transform.py optax/_src/alias.py optax/_src/lbfgs_test.py
• ruff check .
• flake8 --select=E9,F63,F7,F82,E225,E251 --show-source --statistics
• pylint module, test, and quote-consistency checks
• pyrefly check optax/_src/transform.py optax/_src/alias.py
• pytest -n auto --pyargs optax
• make html -C docs
• make doctest -C docs

[Trighap52]

I looked into the failing jobs.

Pytest 3.10 on ubuntu-latest jax=newest is related to this PR. It fails in:

LBFGSTest::test_against_plain_implementation3
problem_name = 'himmelblau', scale_init_precond = False
ACTUAL:  [234.10304, 179.07068]
DESIRED: [234.02055, 179.15878]

I reproduced this locally with jax==0.6.2. The L-BFGS curvature guard itself is working; the mismatch appears to be a compiled lax.while_loop float32 numerical divergence in the existing plain-implementation comparison after the first tiny positive curvature pair.

Would you prefer that I open a small follow-up PR to relax the tolerance for this comparison while keeping the new regression test strict about preventing negative weights_memory?

The Build and check types with pyrefly failure appears unrelated to this PR. The reported errors are in untouched upstream files such as optax/_src/linear_algebra.py, optax/_src/linesearch.py, optax/perturbations/_make_pert.py, and optax/projections/_projections.py; none are in the files changed here.