Fix logical sharding resolution in NNX

[xibinliu]

Description

In pure NNX training runs, model variables retrieve physical PartitionSpecs via get_nnx_named_sharding_with_scan_axis in maxtext_utils.py. Previously, this helper used Flax core SPMD's from_sharding_rules to map logical names to physical axes. However, from_sharding_rules resolves rules by converting the rules list into a dictionary (last-write-wins). This caused fallback rules sharing the same logical name (e.g. 'embed') to overwrite preceding specific rules, dropping essential axes like fsdp_transpose and leading to unsharded parameter percentage assertion errors.

Additionally, resolving specifications independently for each dimension without tracking assigned axes could bind a single physical axis (like fsdp_transpose) to multiple positional dimensions of a tensor, causing DuplicateSpecError.

To fix this:

1. Replaced from_sharding_rules with a Rules-first resolution loop that matches rules sequentially (first-match-wins), matching Flax Linen's mapping behavior.
2. Implemented an assigned_axes tracker within the loop to ensure physical mesh axes are bound to at most one dimension per tensor.
3. Added unit tests covering sequential matching (first-match-wins) and duplicate physical axis prevention during resolution.

Tests

Log with Gemma3-12B (2x v6e-256)

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/utils/maxtext_utils.py	78.78%	4 Missing and 3 partials ⚠️

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