feat(jax): consume the common optimizer configuration

## Summary

JAX training currently constructs a fixed Optax Adam chain and does not consume the normalized top-level `optimizer` block. Optimizer choices and parameters accepted by the common schema can therefore be ignored.

HybridMuon for DPA4 is tracked separately in #5771; this issue covers the general optimizer configuration and dispatch contract.

## Scope

- Read and validate the normalized `optimizer` configuration.
- Add a JAX optimizer factory for the optimizer types that JAX can support.
- Wire optimizer-specific parameters into the selected Optax transformation.
- Fail explicitly for unsupported optimizer types instead of silently using Adam.
- Preserve optimizer state through checkpoints and restarts.
- Keep learning-rate scheduling separate from optimizer selection.

## Acceptance criteria

- Selecting a supported optimizer changes the transformation actually used by training.
- Supported optimizer parameters affect deterministic one-step update tests.
- Unsupported schema variants produce an actionable backend-specific error.
- Adam's current behavior remains covered by regression tests.
- Checkpoint/resume restores all optimizer state and reproduces the next update.
- The factory provides the integration point required by #5771 without duplicating configuration parsing.

Refs #5756.

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh