Clarify and improve scan_layers mismatch error handling

[shralex]

When scan_layers is inconsistent between checkpoint creation and training, Orbax throws an error.
This PR documents this and improves error handling across all possible restore paths.

FIXES: b/520780284

Tests

Unit and integration tests were added to verify error handling.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 71.92982% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/common/checkpointing.py	74.07%	11 Missing and 3 partials ⚠️
src/maxtext/utils/model_creation_utils.py	33.33%	1 Missing and 1 partial ⚠️

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