Allow identical config overrides and handle configuration exceptions …

[shralex]

…cleanly with code 2 and Kubernetes termination logging

This PR addresses two issues in MaxText's configuration initialization and validation:

1. Previously, calling a model config with a command-line override would hard-reject the execution if the key was defined by both, even if the provided override value was completely identical (such as tokenizer_type="huggingface" when running qwen3 models). This PR modifies validate_no_keys_overridden_twice to compare the resolved values. Overrides are now allowed if the override value matches the model configuration value exactly. A ValueError is only raised if the values differ and override_model_config=False.

2. When a deterministic config validation error occurs during startup, JobSet/Kubernetes would previously crash-loop and exhaust maxRestarts because config validation errors looked like infrastructure/general flakes (exiting with code 1).
   This PR catches configuration validation errors, prints a highly visible error banner to sys.stderr, writes the specific failure message to /dev/termination-log for Kubernetes status propagation, and exits with standard exit code 2 (reserved for CLI syntax and config errors). Standard behavior is bypassed when running in test environments (pytest/unittest) to allow original validation exceptions to propagate for assertions.

Tests

Tested on TPU VM

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

✅ All modified and coverable lines are covered by tests.

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