Replace 'with mesh' with 'with jax.set_mesh(mesh)'

[SurbhiJainUSC]

Description

This PR replaces the usage of the with mesh context manager with the newer with jax.set_mesh(mesh) API across the MaxText codebase. This change is being made to address JAX deprecation warnings.

Fixes: b/470443463

Tests

SFT:

python3 -m maxtext.trainers.post_train.sft.train_sft \
model_name=llama3.1-8b-Instruct \
load_parameters_path=gs://maxtext-model-checkpoints/llama3.1_8b_instruct/2025-10-16/scanned/0/items \
run_name=$RUN_NAME \
base_output_directory=$BASE_OUTPUT_DIRECTORY \
steps=2

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 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../trainers/post_train/distillation/train_distill.py	0.00%	1 Missing ⚠️
src/maxtext/utils/lora_utils.py	0.00%	1 Missing ⚠️

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[richjames0]

lgtm